There's apparently a mod that lets you have Muffalobears, if you want. On the topic of Thrumbos, there are "Animal Self-Tamed" and "Random Animal Joins" events. If I remember well, someone discovered, digging though code, that animals with higher Wildness are more likely to get picked by this event. And Thrumbos have the highest vanilla Wildness.
I'd like to thank y'all for the advice on mods. I've picked around 40 that I definitely want to use, and that's probably enough to start a new game. Now I just need to send off my existing colony into space. Once that happens, I can amuse you with stories of how it all went. Text-only, I don't think there will be pictures.