Whenever a Stratta nest is threatened, the Warrior caste/evolution is quick to make themselves known. No, Myrmeshale doesn't have three heads; rather, this is a sort of Phalanx formation where a group of the ants burrow to the surface, form a protective soil barrier, and begin retaliation against invaders with a combination of razor-sharp appendages, slicing mandibles, and acidic spray. These formations of Pokémon are normally made up of three Myrmeshale, but can have up to nine of them, with the largest of the Warrior-caste acting as the "commander", normally denoted by a more impressive crystalline "crown".

Myrmeshale warriors evolve from the Stratta workers of the colony whenever enough of certain rare minerals are fed directly to the smaller, less combat-ready Pokémon, but can also evolve as a simple adaptation to frequent exercise and combat. Scientists don't really understand the exact catalyst in the wild, as there seems to be no common link between the minerals consumed and the development of the Myrmeshale's more combat-focused form.

Myrmeshales are roughly the size of a small dog, and extremely heavy. They're remarkably durable, and upon evolution learn a unique ability known in Trainer circles as Entrench, which is when they form their combat groups for both attack and defense. It has thus far been impossible to catch only a single Myrmeshale, as they spend their entire lives in a combat-ready state, close to their kin in the nest. Much like a Dugtrio, the grouping of Pokémon is considered a single being for the sake of discussion.

Myrmeshale, the Phalanx Defender Pokémon, No. 006 in the Ōshū Global Pokédex.

Art, once more, by the talented and generally great  GoatMystic