equus

Equus, equi is second declension masculine and entirely regular. The mare is equa, first declension. Ablative equo does the work of "on horseback", contrasted with pedibus, on foot.

Two things to keep straight. Eques, equitis (rank 628) is a different noun built on this one, and it means both the cavalryman and the member of the equestrian order, so equites in a battle narrative are horsemen and in a political one are a social class. And equus has nothing whatever to do with aequus, "level, fair", despite the near-identical look; equal and equine are strangers to each other. The other surprise is what happened next: equus did not survive into everyday Romance. The army's slang word caballus, roughly "nag", took over, giving French cheval, Spanish caballo, Italian cavallo. English keeps equus only in the learned words equine, equestrian and equitation.