You could try unlocking them by moding.

Find export_descrption_unit.txt file (in data folder).
Backup it, just in case.
Find in it cheat oliphants.
in line with attributes delete ", no_custom"
and in the line ownership add which faction would you like to have it.

This, way, that facion would get them in custom battles.

For a campaign play, it's a bit more difficult...