Planeswalker Challenge - Monsters' Advocate (Casual)

Created for the Planeswalker Challenge - build a deck for any Planeswalker that is thematic, fun to play and accessible.

The simple rules of the challenge:

1. Choose your Planeswalker.
2. Your deck must include 3 copies of that Planeswalker (or, for those that already have theme decks, you can include 1 copy of that Planeswalker and the 2 cards from that deck that tutor that Planeswalker).
3. Your deck cannot include other Mythic cards, and can only include up to 4 Rare and 16 Uncommon.
4. Your deck can only include cards from the same block (or set, post-blocks) as your Planeswalker.

The guidelines for the challenge:

1. Try to create a deck that really shows off what your Planeswalker can do, and that suits their personality.
2. Try to stick to a colour identity that is thematic for your Planeswalker.
3. If your Planeswalker is aligned with a particular faction, try to include only cards from that faction and 'neutral' cards, not cards from other factions.

About this deck:
Vivien loves creatures, a lot. She lets you cast creatures from the top of your library, she can create a 3/3 Beast with an ability token each turn, and she lets you cast creatures 'buy one, get one free'. So clearly we want a deck with lots of creatures in, and bonus points if we can find creatures that fit thematically with Vivien and her abilities.
The result is a very creature-heavy deck - 28 creatures in all. We have a few mutating creatures: Glowstone Recluse, Gemrazer and Auspicious Starrix, the first two of which come with abilities similar to Vivien's own Beast tokens, and the third of which lets you 'cheat' another permanent onto the field each time it mutates. To ensure we make the most of their mutating abilities, we've got some nice creatures for them to mutate onto – Ivy Elemental can be as big as you need it to be, Essence Symbiote makes mutating better value, and Fertilid lets you trade +1/+1 counters for ramp. I've also included a few creatures outside of this mutation scheme – Humble Naturalist gives you ramp, Hornbash Mentor gives you trample and +1/+1 counters, and Kogla, the Titan Ape is a big smashy titan ape.
We only have a few other cards, all of which are thematic inclusions – Vivien is quoted on Sudden Spinnerets, which doesn't stop it being a nice answer to pesky flying creatures, and is pictured on Thwart the Enemy, which is a game winning card when used at the right time. Finally, Lead the Stampede both features Vivien in the artwork and is excellent card draw in this deck.
Although I did try to include the Vivien themed cards, I just couldn't find space for Mosscoat Goriak.