People can already use the support elephant in the Mauryans civ to do that. I don't know if there is other units that can do that... So, we can let the user choose if they want to play with the Mauryans to have the support elephant or we can add an equivalent unit in each civilisations.. It's not too hard to code. We do a copy of the elephant template, but we change the visual actor and some attributes like name, description, etc. Anyway I think it's better to not do that. Because with only the Mauryan's support elephant, that give a different game style depending on the civ the user choose.