Okay, I think I found the answer. 'Eponymous' means named after a person, 'toponymous' is named after a place, so a word named after the form of an object must be 'morphonymous'. It's not in any of my dictionaries but it's okay to coin new words I guess. 'Morph' comes from the Greek for 'form' as in metamorphosis. 'Onoma' is Greek for 'name' -as used in the other words I mentioned.
I have found one use of 'morphonymous' on the net (Google it yourself maybe) so either they coined it themselves or they found it somewhere.