Gunpowder is Ichabod Crane's horse.  The old steed has clearly seen better days.  Iriving writes:

He was gaunt and shagged, with a ewe neck and a head like a hammer; his rusty mane and tail were tangled and knotted with burrs; one eye had lost its pupil, and was glaring and spectral; but the other had the gleam of a genuine devil in it. Still he must have had fire and mettle in his day, if we may judge from the name he bore of Gunpowder.

Crane's horse may well be a reflection of the man himself:  past his prime a bit short-sighted.  Like his horse, Crane still has some "genuine devil" in him, as his actions reveal.    

Irving describes Gunpowder as "a broken-down plough horse...gaunt and shagged, with a ewe neck and hammer head...his rusty mane and tail were tangled and knotted with burrs...one eye had lost its pupil and was glaring and spectral, but the other had the gleam of a genuine devil in it".  Crane, when astride the horse, is a comical figure, plodding along on the tired beast, with his stirrups too high and his bony elbows and knees sticking out at all angles.