As for *h1ors-o-s, it's attested even in Hittite.
>
> Piotr
I'm going into folk etymology, so just slap me down where I'm wrong.
H1ors ... any potential relation to 'horse'? If only something
internal to English or Germanic?
The backside of a horse is proverbial, as with "a horse's ass". I'm
thinking perhaps something happened along the lines of 'caballus'
which apparently (in Celtic) originally meant 'nag', 'old, tired
workhorse'.
EIEC reports an Old Irish sense, 'back of chariot' (presumably, the
rear of the vehicle itself).
Since the charioteer always looked at the horse's backside, the idea
that Germanic might have called the horse a 'backside' not
semantically improbable.
'Ass' in the sense of donkey is suggested to be a Celtic borrowing
into English, but deriving ultimately from Latin. Latin 'asinus' is
probably borrowed
Mark.