TB = Thoroughbred and gelding = castrated horse, usually a male.
Here is a TB Thoroughbred named Lincecum. I don’t know why he’s named Lincecum 2008 or what the 2008 stands for, but that’s exactly how I found this video: looking for Tim Lincecum video highlights from 2008.
Be warned: if your YouTube settings are such that the next video autoplays, know that the next video it recommends following this one is a how-to on gelding a horse. So, if you want to watch a horse castration when you really just came here for baseball stuff... go for it.
I’m talking about the man, not the horse, of course.