Last year for Christmas my mom gave Peter a toy rocking horse that made "neigh" and "clopping" sounds and moved its head. Peter was terrified of it, but Aaron always loved it.
Peter has since come around to the horse, which now lives in Aaron's room. Recently we asked the boys what the horse's name was and Peter announced that he was called "cow"!
I get the feeling that Aaron was the one who made up the name: