Here's a screenshot. You can see it in The Last Jedi in the brief underwater shot when Rey falls into the dark side well. It's immediately preceding the famous "mirror Reys" scene; on the Blu-Ray, it's at the 1:13:56 mark.
I was unable to find any reference to what that skull, very much resembling a T-Rex, lying at the bottom of the pool, might be. This isn't something that can get into a shot by accident, it had to be designed and placed, so there must be some background to it.
So, what's the in-universe and out-of-universe story on that skull?