I confess that I have never seen images like that -- they are usually of the warchariots of Vishnu or Krishna (one problem is that when the Hindus took over Buddhist images they began to treat the Buddha as an avatar of Vishnu) -- but if they do exist from the earliest period I would assume that they are likely to be the Buddha attacking underworld demons. I wouldn't mind a reference if you can find one -- I always like disconcerting my Buddhist colleagues who are always going on about how pure and gentle we have always been. Hah.
yours, Peter T.