I've used keyboards with the trackball mounted on them. In that application I've seen them called thumb balls. With repeated use, I got a big knot on the top of my thumb. The regular-sized trackball allowed me to use my entire hand to roll it.
I haven't seen them on a keyboard, although I have seen some laptops that used a ball rather than a trackpad. I think they were old, though.
Some of the responses verged on outright hostility. I don't think Beth likes me anymore. (Beth replies "What do you mean by anymore? I never liked you from the start.") I've learned my lesson. No more helpful suggestions. Btw, I tried using my thigh as a mousepad but it gets me all excited. Maybe I should try lower down.
About a mousepad, specifically, and I was joking. You can tell when I make a joke because it's never funny.