Scuttling Sentinel is a uncommon Creature — Crab Elf card from Ravnica: Clue Edition (CLU), collector number 42. Current market price: $0.26, TCGplayer market prices via Scryfall.
Flash Vigilance When this creature enters, put a +1/+1 counter on another target creature you control. Until end of turn, that creature becomes a blue Crab in addition to its other types and gains hexproof. (It can't be the target of spells or abilities your opponents control.)