Total War: Warhammer III

This has a little trick to it. It did not unlock for me, no matter what kind of mount I put a character on across multiple factions. I tried getting three of the same type, three different types, didn't unlock no matter what mount or faction. I only unlocked it after taking the characters of the mounts and as such putting the mounts into my factions "inventory". It unlocked right away after that. You can put the heroes back onto the mounts right afterwards.