Divinity 2: the Dragon Knight Saga. Fucking brilliant game too (although some of the combat styles do suck)
Word of warning if you're interested in this game just because of the ability to change into a dragon: when you finally get to turn into a dragon you'll find that your ability to enjoy this faces major obstacles. In many areas of the game there are very low glass ceilings that limit your ability to fly. Some of these are even lower
than the thing you're trying to destroy, or lower than the entrance to the area you want to visit (walk in, fucko!). All normal enemies also disappear the moment you transform, so you're limited to breathing fire at turrets or the occasional enemy dragon thing. Most places aren't very large either, so there's not much room to fly around, and the ones that do have space are barren and empty.
I'll admit that there's some games that I've played purely because they had dragons/dinosaurs and/or lizard people in, because I enjoy that sort of thing, but even I couldn't bear to play through that game more than once. Yes, you can turn into a dragon, but if you're only going to play it because that's an option, you're in for crushing disappointment. Also I remember the combat being incredibly boring tedious bullshit with obnoxious levelling mechanics (in that enemies 3+ levels above you are instant death, and 3+ levels below are made of paper and give 1xp) that forces extremely linear progression despite the apparent openness, but that's another thing entirely. Maybe I just had a shit build or something? I don't know or care enough to find out.
Adding to the list, there's an (apparently not very good) MMO called Istaria where you can play as a dragon. 15 years old now (so I guess it can't be
that bad, right?). There's also a dragon faction in Battle for Wesnoth. And you can kinda-sorta turn into a dragony thing in Dark Souls, though it's more of an anorexic goat in the third one. Plus 2 or 3 pretty awful games on Steam whose names I forget. There's also Divinity: Dragon Commander where you can turn into a jet-powered dragon (seriously) to help out in the RTS sections for a limited amount of time. Oh, there's also the Guncaster mod for Doom (2) where you play as a dragon dude (which incidentally has possibly the best shotgun of any game ever). The base game is pretty OP and even demolishes horde maps pretty well, but there's an alternate version called Guncaster Vindicated which is more of a challenge.