Keen Smell. The giant ant has advantage on Wisdom (Perception) checks that rely on smell.

Hold Breath. The hippo can hold it's breath for 10 minutes.

Multiattack. The giant ant makes one bite attack and one sting attack.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 19 (4d6 + 5) piercing damage. If the target is a Medium or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the creature is restrained and the hippo can't bite another creature.

Sting. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 9 (1d8 + 5) piercing damage plus 22 (4d10) poison damage, or half as much poison damage with a successful DC 12 Constitution saving throw.