Hive Tyrants: Vessel of the Hive. Appears as a normal pony, but can mutate pretty much on whim. Has parts of their original personality, but, as their name suggests, are mostly just The Hive-which is made of EVERY personality.
Carnifexes, Trygons and Other Big Things: Pretty much the same as warhammer canon, or some other large scary monsters.
Everything in between: Ponies, but often with added spikes.
I'm drawing on tyranids with a little bit of Borg here, and adding some random other bits cause I feel like it.