John Rees-Evans, the party's parliamentary candidate for Cardiff South and Penarth, made the comments in response to a protester who was questioning statements by his fellow party members.
In particular, he was responding to quote by a UKIP member who claimed that "some homosexuals prefer sex with animals."
Rees-Evans said his colleague was right. He said: “Actually, I’ve witnessed that. I’ve got a horse and it was there in the field.
"And a donkey came up… which was male, and I’m afraid tried to rape my horse.”
Rees-Evans said he then had to hit the donkey to protect his horse. His remarks were captured on video at a protest outside a campaign office
He told the Independent: “For me, any kind of non-conventional relationship that people have, I would say I don’t really understand the attraction, but we are a libertarian party."