Horses are so important to human history that it's no wonder English speakers have come up with all sorts of interesting terms to refer to them. Especially in Britain. Here are just a few:
Gigi or GG
Mr. Ed (from the TV show)
Beast or beastie
Bottle of sauce (Cockney rhyming slang)
And those are just the ones for ordinary horses. Here are a few colorful terms for "useless" horses:
Moose (usually for a big, clumsy horse)
(And any variant on the first and last. I remember growing up angry riders threatening to send their horse "to the hunt" - which I think needs no explanation).
Oh, and a longears is a mule (or sometimes a donkey).