Nellisir
Hero
It's an interest of mine. The status of some are unclear, but I only included the ones that have a fair degree of confidence.I was thinking of neanderthals and us. I'd forgotten about the denisovans, I've only read a single article on them. Not even sure if I had heard of the others. Thing is, that is still only a relatively small amount when compared with DnD fantasy worlds. Even if you just stick to the players handbook races, you have more than that. I don't find it very realistic to include dozens of races but then, like I said, I will just shrug and go "It's fantasy."
Keep in mind there are more - possibly many more - that we won't ever know about. An isolated population that left no physical trace for us and didn't cross back into our lineage in any quantity is gone. You need remains that have some trace of DNA to analyze, and many areas just aren't suitable to preserving remains or DNA for very long. And you need to find it. Anything currently underwater is gone. Anything scoured by glaciation is gone. Anywhere too humid or moist or wet. Anywhere without significant cave shelters. Anywhere significantly developed in the modern age.