Bunny rabbit probably...
but that isn't the point.
Cocoa Krispies first hit the shelves in the US in 1958 and came with this cute little monkey mascot names Jose.
The poor guy didn't last long though before he got fired and they gave the job to an elephant.
My own person thought is that a monkey eating little brown things brought other things to mind besides breakfast.
Monkeys are famous after all for flinging poo.