I like the card design for Yin Yang, but feel it's a little underpowered. Perhaps making it a 2/2 and giving it bigger abilities that trigger on casting black and white spells would make it better in play and thematically. For example:
Yin Yang, Never Forgotten {1}{W}{B}
Legendary Creature - Cat
Deathtouch,
LifelinkWhenever you cast a Black spell, if Yin Yang in in the command zone or on the battlefield, create a 1/1 white Cat creature token, then each opponent loses 1 life for each Cat you control.
Whenever you cast a White spell, if Yin Yang is in the command zone or on the battlefield, create a 1/1 white Cat creature token, then you gain 1 life for each Cat you control.
2/2
I like this design more because its triggers on all Black and White spells instead of just cats, making it more powerful. On top of that, it further embodies the Yin Yang in gaining life and losing life abilities. It also opens up a couple more routes. You could head down a lifegain-synergy deck, a token synergy deck, or just your average Orzhov deck with the usual goodstuffs.
If you want to print your card, you can download it from MTG Cardsmith as a .png and print it from there. Sizing is a little fiddly, but either laser-printing it onto card-thickness cardboard or (the way that I do it) printing it onto normal of paper, gluing 2 sheets onto it, then cutting it out from there makes a passable Magic card. You've gotta be good with scissors though. Alternatively - and probably the better route - you could hire someone else to do it for you, or ask people who know how to do it properly. There are whole communities on Reddit about this stuff.
My condolences for your pet, and I hope these suggestions are useful.