1815, representing U.S. dialectal variant pronunciation of horse, especially as applied to (large or coarse) persons.
Online Etymology Dictionary, © 2010 Douglas Harper
Examples from the Web for hoss
Here he's left Buck's hoss, so he ain't exactly a hoss thief—yet.
I looked back and seen Allister sitting on his hoss, dead still.
That's why he pulled up his hoss and waited for Allister to make the first move for his gun.
Then a gal cum out on hoss back and commenced ridin' around.Uncles Josh's Punkin Centre Stories
I guess that hoss thinks he's goin' to be froze to the ground.Tiverton Tales