[fnord123]

I think it's named after rhyming slang for crap (pony and trap).

[spooneybarger]

also nope:

it's from the phrase "and I want a pony"

[teddyh]

Originally from here:

http://www.gocomics.com/calvinandhobbes/1987/01/13

[spooneybarger]

that would be it