This is strange...

[supersailorpeach]

I have a long crocodile dog toy with a lot of squeakers. Whenever I hold it a certain way, my dog Chewii (the name has nothing to do with Star Wars) "dominates" it. She's female and fixed!  

[witchhunterrobin80]

It is a playing behavior, female dogs do this all the time to show who is boss. I have even seen female dogs mount male dogs on many occasions. It is just the female showing her dominance  

[Citrus Hikaru]

Yeah, some dogs are just dominant like that.
My little dog is the same, and she tries mounting my lab mix! I guess size doesn't matter to her...