Ankheg Hatchling (Swarm)

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Small creature. The swarm can't regain hit points or gain temporary hit points.

Bite (Swarm has More than Half HP). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (2d4+2) slashing damage. If the target is a Small or smaller creature, it is grappled (escape DC 11). Until this grapple ends, the Ankheg Hatchling can bite only the grappled creature and has advantage on attack rolls to do so.

Bite (Swarm has Less than Half HP). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (1d4+1) slashing damage. If the target is a Small or smaller creature, it is grappled (escape DC 11). Until this grapple ends, the Ankheg Hatchling can bite only the grappled creature and has advantage on attack rolls to do so.