My Joli started humping any toy that was her size or bigger. It is a pretty fun story of how it started though. I got her a big pink monkey for her first Valentines day, she had a new Valentines dress and bow and I gave her the monkey...and she almost immediately started humping it. She was only 6 months old and still pretty small. She sniffed it a few times and then started at it. So it seemed like she took her Valentines present a little too literally....
I wouldn't let her play with that toy very often because EVERY time she got it she would hump it, it ended up being named 'Hump Monkey', and sometimes she would go pretty crazy with it *embarrassed face*
. I finally threw it away to try to keep her from doing it, but any time she gets something that is about her size she will make it 'hers'. So I have to make sure all of my stuffed animals are put on shelves not only to prevent chewing but humping. She also tries to hump my leg pillows at times.
And when we got Somi [we have had her for about 6 months] she would try to hump her when they would play. And this is when she would play nice with her and they got along [Joli currently fights with her a lot
]. It was so funny that it was really hard to sound serious enough to get her to stop, with Joli being close to 9 lbs and Somi being barely 2 lbs at the time. And I said some phrases I never thought I would say along the lines of "Don't hump the baby". But she hasn't tried to do this in a long time.
I was also hoping that all of the humping would stop after she got fixed, but it has stayed about the same with the leg pillow and whenever she gets a big toy/toy type thing.