Liquid Ooze

Introduced in Generation III, Liquid Ooze reverses the healing effect of all draining moves. When an opponent uses a leeching move like Absorb, Mega Drain, Giga Drain, Leech Life, or Drain Punch against this Pokemon, instead of recovering HP, the attacker takes damage equal to the amount it would have healed.

This turns draining strategies into a dangerous liability. Leech Seed is also affected -- a Pokemon seeded by a Liquid Ooze user will still lose HP each turn, but the intended recipient takes damage rather than healing. Dream Eater is notably the one exception, remaining completely unaffected by Liquid Ooze.

Found on only 4 Pokemon -- Tentacool, Tentacruel, Gulpin, and Swalot -- Liquid Ooze serves as a hard counter to drain-based recovery strategies and can punish opponents who carelessly use HP-absorbing moves without scouting the target's ability first.