So, so, so many and the list will continue to grow as the old guard gets fazed out and enters their late 40's/early 50's. There was a time when the MMA community prided itself on how their fighters didn't end up vegetables like in boxing. A big reason for this belief was the lack of a mandatory 10 count; if you got badly hurt in MMA a guy could follow up and finish you off quickly whereas in boxing you could get a significant break once you hit the mat, try to gather your wits, and continue to get pummeled for quite some time after. Of course, anyone who has seen the severity of KO's in MMA knows that getting absolutely lamped by a head kick followed by huge GnP before a ref feels inclined to step in, leaving the fighter absolutely KTFO is far worse than damn near anything you'll see in boxing (with some notable exceptions). There are also numerous examples of the long-winded GnP beatdowns that just go on forever despite the fact that it clearly should've been stopped far earlier.
The reason we didn't use to see so many damaged fighters back in the early to mid-200's was because the sport was so young back then that the vast majority of fighters hadn't been doing it long enough to be openly wearing the ill effects of what MMA can do to a man. Now we see guys who were in the sport forever, stuck around too long, and are in really rough shape because of it. It's a brutal contact sport and will always take its pound of flesh from those who choose to participate.
