This plaster cast from a display at the British Museum has recently been installed at Old Swan House.
It has given rise to a discussion on what scene was being depicted. My local classical scholar thinks that it most probably depicts the myth of the Rape of Europa, in which Zeus disguises himself as a bull and carries Europa off to Crete, where she gives birth to the Minotaur among others. This scene must be early in the story as Europa seems unwisely to be treating the bull as a pet. Soon afterwards, he persuades Europa to climb onto his back, whereupon he swims with her off to Crete.
See a painting of the myth, Titian at the National Gallery - Love, Desire, Death.