The offspring will be the lowest-stage evolution of the female Pokémon or non-Ditto Pokémon. If your Tyrantrum is female, you can go ahead and start breeding with Tyrantrum and Aggron. If your Tyrantrum is male, then you have to get a Ditto and breed it with that. Masuda Method will work with both cases (as long as your Ditto isn’t also Japanese, if you use one).