Slithering Cryptid is a common Creature — Fish Mutant card from Teenage Mutant Ninja Turtles (TMT), collector number 168. Current market price: $0.07 (foil $0.17), TCGplayer market prices via Scryfall.
When this creature enters, create a Mutagen token. (It's an artifact with "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")