Vine Mare

The whole design is a closed loop pointed at one color. Hexproof shuts off removal from a black opponent's hand, and the evasion clause shuts off black blockers, so against a deck built on swamps the 5/3 body simply cannot be answered in either of the two ways black usually answers a creature: it can't be targeted by removal, and it can't be chumped by the spider or zombie standing in front of it. That two-pronged hate is the trick. Hexproof alone is generically useful; "can't be blocked by black creatures" alone is a narrow color-hoser. Stacked together they make a clock that one specific archetype has almost no clean tools against, precisely the asymmetric pressure aggressive green wants in a metagame leaning on black control. The cost of that focus is the toughness. Three is fragile to anything that doesn't need to target (combat, edicts that sacrifice rather than destroy, board sweepers that hit toughness), so the protection it does carry is loud about what it doesn't cover. It is a member of a small mare cycle, each a hexproof body that can't be blocked by one enemy color, and the green one is the most aggressively statted of them: the largest power, the lowest toughness, the bluntest expression of the idea that this creature exists to attack a black opponent to death and nothing else.