RTS where you can create an AI ally mid-game?

Hi all,

I've been reading an excellent sci-fi book recently (the bobiverse by Dennis E Taylor) and it made me think it would make for a pretty good RTS game. I don't think it exists but I'll ask in case I missed it:

Usually you start skirmish games by choosing the teams first, choosing how many AIs play against or with you. Some games allow you to change the teams in-game but as far as I know, none let you create an extra player during the game.

Is there a game in which you can, for instance, create/build a unit that once finished becomes a permanent AI ally, autonomous?

I can easily imagine scenarios very suited for such system, have an adversary grow very strong very fast which could not be overcome alone. The player would have to play smart to have the resources required to build an AI which would then join the fight and alleviate the pressure somewhat.

Probably doesn't exist but it's also an opportunity to share a good book :)