Hydra Tribal

Note that on EDHREC, there's only 1 deck of Reyhan & Gilanra partnered together - and that's presumably this deck. So it you want to be hip and cool and unique, here's a deck to really stand out from the crowd :)
That said, if you want EDHREC suggestions, look up Reyhan + Slurrk All Ingesting, as that's a similar +1/+1 theme. That has currently got 107 decks on the database - https://edhrec.com/commanders/reyhan-last-of-the-abzan-slurrk-all-ingesting

'Budget' Deck

This deck started off life as a somewhat-budget deck. The most expensive pieces - Mana Reflection, Toxic Deluge, Primordial Hydra - I already had spare from decks that were torn down / got cut from other decks for not quite being the right fits. 10 months after making the deck, the price drifted upwards about 20 dollars - owing to what has and hasn't been reprinted I suppose - but taking out Mana Reflection (which isn't worth the price in all honesty, it doesn't stick around long enough to get its value out at 6 mana from my expererience), Toxic Deluge and Primordial Hydra (last printed in M13) will save you $52 (card Kingdom, June 2022), and set the deck at a $98 Card Kingdom. Noting that Card Kingdom has a minimum of 25 cents for any given card - and Snakeskin Veil is only a 4 cent card normally - it gets a bit weird, I don't know, I win most of my cards off auctions. Heck, I'm in Australia, we don't use Card Kingdom in general, so $98 US is literally a foreign concept to me.

Deck Plan

Ramp Hard Go Hard

The goal of the deck is to ramp out hard, and to cast big X spells so that no matter the state of the game you're in, you can put either 6 or 10 mana into one of the hydras. The ramp and draw from Gilanra, Caller of Wirewood means you always have access to a 3 mana ramp drop, and she's relevant on later turns as she helps find more cards when you're casting big creatures. For X cost hydras on the stack - for example, Mistcutter Hydra for (X)(G) , if you pay X = 5, it's a 6 mana value/ CMC 6 card - and Gilanra can draw off of it.

Not-Actually-Ramp Ramp Pieces

Note that there's allegedly 16 pieces of ramp, but 5 of them aren't normal kinds of ramp with how they play. Not featured in the ramp part of the list are Gilanra, Caller of Wirewood (who is in the Sideboard and is actually ramp as a 3 mana dork) and Uvenwald Hydra, who at 6 mana fetches up a land. That's great, it's not really acceleration into the mana curve though. Mana Reflection is 6 mana now to get 12 mana all at once next turn for a very big Hydra, so it's not really ramp either (was cut for Vigor, which is protection on attack that directly synergises with +1/+1 counters). Kodama of the West Tree cares about there being a board state - so there has to be at least 1 other card on the table before she (it?) does anything. Thankfully, Reyhan is in the command zone, but they're both 3 drops and usually Gilanra will come out before either of them - situations change though, I've never drawn Kodama of the West early game to say that it's best to play Reyhan turn 3, then Kodama turn 4 and Gilanra turn 5. So it's all armchair theory crafting here. Gargos, Vicious Watcher is amazing value when it comes down, but it only helps out other hydras for mana (but 4 is huge!). Also, the fight and vigilance on a 6 mana 8/7 body is damn good value. Even if he only saves you 4 mana once, that's still pretty good.

Rampant Rejuvenator is weird. I've played it only once so far in a real game (100 card singleton no creature tutors), and it got up to 6/6 with a corpsejack Menace on the field and an Evolution Sage on the field. And nobody wanted to kill it. At 2 power, people didn't seem to care. But at 6 power, people refused to kill it. The Jeskai player combo'd off and won that game, but it was an interesting response of fear for people not wanting to kill it. So it never actually ended up ramping me at all.

Card Draw and Protection

Reyhan is there as a bit of protection - my pod likes single target removal, so he makes it difficult to totally set the board state back on me. Much the same, the Protection section generally gives protection to 1 creature so in the case of a board wipe, one creature survives and gets any and all counters that have been accrued over the game. It doesn't save the board from all board wipes - a Terminus, Merciless Eviction, Toxic Deluge, etc. - will still stop us in our tracks, but so be it.

As such, him being able to move the +1/+1 counters around is solid fun, and can lead to him becoming big enough to deal significant commander damage as a pivot strategy when plan A of trample Hydras goes badly. Alternatively, drawing cards with Hunter's Insight, Momentous Fall, Return of the Wildspeaker and Greater Good can lead to very large hands. It's a feels-good when you draw one of those cards and have Reliquary Tower out.

Which Hydras Got Chosen

I'm not going to lie, some of the hydras in here are just because I think they're cool and not because they should be here. Polukranos, Unchained having a fight ability has been niche all of once. It was pretty cool, but I like Greek mythology aight aha. Ironscale Hydra is in the same vein, it's cool but hard to get +1/+1 counters onto. Weirdly indestructible, which had been relevant once in 10 months until one game in which it was MVP and stalled out an agro Isshin, Two Heavens as One deck (only a 2 player game, I don't remember what happened to the other 2 players).

Grackmaw, Skyclave Ravager I will die on a hill for, it's actually awesome in the deck. It cares about dying triggers, makes another body if it dies (and Reyhan can get double value out of those counters moving around too!). Capricopian has been relevant once, watching 2 players drain their mana to avoid being tagged by a 10/10 was fun politics. I really should play this deck more often, it's good fun.

There's only 8 cards that are (X)(G), most of them have fixed mana values. This could go higher by 1 or 2, I did prioritise as (X)(G) and (X)(G)(G), I didn't want to go higher, but they're flexible in nature so perhaps something could be cut for 2-4 more like Ironscale Hydra, though there is a certain density of fixed cost hydras for Genesis Hydra to hit off of.

Trample Time

There's 6 cards that give all (relevant) creatures trample (and a 7th one that gives menace). They are;

• Kodama of the West Tree
• Rhonas's Monument
• Gnarlid Colony
• Hagra Constrictor (menace)
• Pridemalkin
• Garruk's Uprising
• Overwhelming Stampede

Hydras don't get a whole lot of trample or flying or (weirdly enough), reach (only Uvenwald Hydra in this list has reach). So getting through chump blockers comes down to trample most games. These are important sign post cards to play. A few hydras do individually get trample, but getting hydra to everything is important.

Best Cards in the Deck

Corpsejack Menace the true MVP

Corpsejack Menace while out doubles up all the counters from Reyhan moving counters off of a dying creature. With Corpsejack out, play Reyhan? He's now a 6/6. If a hydra with 8 +1/+1 counters on it dies - Momentous Fall for example at instant speed - you can put 16 +1/+1 counters onto Reyhan. He's now a 22/22, and that's very lethal commander damage if unblocked. It's very easy to see a few things die with Corpsejack and Reyhan out and see things spiral for your opponents. Getting those two out plus Greater Good is mortifying as you can draw upwards of 20 cards at instant speed as needed. But the fun needn't be big combos - casting a hydra for X =5 and making it a 10/10 for generic good value is just good value. As a minor note, Solidarity of Heroes does the same thing but as an instant and one-off effect. It doesn't scale nicely, it's 2 mana per creature it targets, but that's just what it is. Doubling Season would be good, but that's a very expensive card that I don't own, and I like to have this deck as one of my weaker decks to just jam when I'm at my LGS and that's just what the table is up for on a given Friday night.

Gargos, Vicious Watcher

Huge mana reduction, and he fights as an 8/7 when people try and pick off your big threats with single target removal - Swords to Plowshares, Unsummon, Terminate - he doesn't care. He just beats in your opponents best thing's face. Seedborn Muse? Gone. Ni-Mizzet Parun? Goodbye. All for 6 mana.
Keep in mind that he also triggers off of your own spells that target creatures you control. You can Kaya's Ghostform or Solidarity of Heroes your own creature. And then fight another creature of your opponent's which is just good value.

Deck Building Considerations

The Maybeboard

In regards to the Maybeboard, you'll notice the oozes. At one point, this was a generic +1/+1 counters deck, but I turned it from including Oozes to being more tribal. The main remnant of that is Slurrk, All-Ingesting, who was the original partner commander with Reyhan. Gilanra being a source of ramp worked better than Slurrk, who doubles up with Reyhan and would work as an incredible aristocrats duo, but that's not quite what this deck is about. There's not enough graveyard synergy, it gets lots of +1/+1 counters the old fashioned way of big mana. It's got the card draw of Greater Good and Monumental Fall which are wonderful, but not the way to back that up.

Tokens Section - Upgrades & Downgrades

In the token section is Mana Reflection, Steelbane Hydra and Incubation Druid. Both are outstanding options, and Mana Reflection worked wonderfully when it was in the deck, but it's been switched out for Vigor (both are 6 mana, but Vigor is more on-theme for +1/+1 synergies. Both are 'generic good green stuff', both kinda work for the theme, but more on theme for +1/+1 counters than to have more mana for X cost Hydras). Neither are bad options, Mana Reflection is slightly better on the wallet and it works really well, I just found it awkward at times to play it out. Steelbane Hydra was hyper expensive when I built the deck, Eldraine was its swing and Steelbane was $10 US. A good card, but not worth that price for removal because it was okay in standard or something. It's now less than a dollar since it was reprinted in Kamigawa Neon Dynasty Commander, but I haven't seen one so I haven't picked it up just yet. Incubation Druid is there because I tried her out, but I wasn't quite getting the reliable +1/+1 counter to make it work. I generally prefer land ramp, and Rhonas' Monument being the exception because it gives trample (which is really important for big hydras, as most of them don't have trample). Migration Path, Oran Rief Hydra were cut as part of a revision about 8 months after the initial bout of playtesting mid-late 2021, and were replaced with 2 new cards from Kamigawa Neon Dynasty (Kodama of the West Tree and Rampant Rejuvinator) and Increasing Savagery.
Barrier Breach could be changed out for Beast Within, but I wanted a way to get through indestructible enchantments - such as god cards, which can be problematic in my pod's meta at times. Not a lot of interaction with other decks unfortunately, it's a bit battle-cruiser as it is.