Undertale not only credits Temmie Chang, it has characters based on her in the game.

(There may be others credited; Temmie's the only one I can think of off the top of my head.)

Also not so sure that it's a billion-dollar game.

It's certainly made a ton of money, and has generated multiples worth of that in fan content, but that's partly because Fox has been so liberal with policing trademarks/copyright.