Thanks for your replies. One more thing tho'. Why the action between them that I witnessed. It has left me amazed, thinking perhaps nature was sending out another branch of her tree.
The rabbit humped the Guinea pig, right?
Have you ever seen a dog hump someone's leg, or hump a cat or another animal?
Rabbits are sexual animals, that's why we joke about "breeding like rabbits". If they're not neutered they will hump anything that's available when the mood strikes. In this case it was the guinea pig.
Neutering your rabbit will help.
No, nature wasn't sending out another branch of her tree, these two animals aren't even in the same forest.