Toby F.P.

He's Bert's best doll friend. Bert named him, himself.

His name is Toby F.P.

He used to be a girl, but Bert doesn't know that.
Apart from that, the two are alike in almost every way.
Can you guess what Toby F.P. stands for?
I don't know if I should tell you. It is not very polite, but neither is Bert, and I know some of you have heard stories (they are all true).

(F.P. stands for Farter Pants.)