I've seen Banjo hump dogs before occasionally. Actually, mainly this JRT x we've now been minding for 2 weeks.
Tonight, after they'd been playing really wildly for a while, she started humping him and got really "into the zone". It took her a few seconds to get off him after he started snapping at her in anger. And when she got off him, she was humping the air for a bit!
I though the humping was just a dominance thing. I'd never seen a female get so carried away with it? As if it really was an instinct she couldn't control.
It was interesting trying to explain it to my 7yo daughter...