Watchful Radstag is a rare Creature — Elk Mutant card from Fallout (PIP), collector number 335. Current market price: $2.71 (foil $2.83), TCGplayer market prices via Scryfall.
Evolve (Whenever a creature you control enters, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.) Whenever this creature evolves, create a token that's a copy of it.