Ankhimera

Three-Headed. The chimera has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious. 

Multiattack. The ankhimera makes three attacks: one with its ankheg head's bite, one with its dragon head's bite, and one with its horns.  When its acid spray is available, it can use the acid spray in place of its ankheg head's bite.  When its cold breath is available, it can use the cold breath in place of its dragon head's bite.  However, it can't use both acid spray and cold breath on the same turn.

Ankheg Head's Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6+4) piercing damage plus 3 (1d6) acid damage. If the target is a Large or smaller creature, it is grappled (escape DC 14). Until this grapple ends, the ankheg head can bite only the grappled creature and has advantage on attack rolls to do so.

Dragon Head's Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 15 (2d10+4) piercing damage plus 4 (1d8) cold damage.

Horns. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 10 (1d12+4) bludgeoning damage.

Acid Spray (Recharge 6). The ankheg head spits acid in a line that is 30 feet long and 5 feet wide, provided that it has no creature grappled. Each creature in that line must succeed on a DC 15 Dexterity saving throw, taking 21 (6d6) acid damage on a failed save, or half as much damage on a successful one.

Cold Breath (Recharge 6). The dragon head exhales cold in a 15-foot cone. Each creature in that area must succeed on a DC 15 Dexterity saving throw, taking 31 (7d8) cold damage on a failed save, or half as much damage on a successful one.