In The Flash: Eobard Thawne is the reverse flash. It is revealed that he went back in time to kill Barry's mother and then was stuck in the past. He kills the actual Harrison Wells and take his form and waits for Barry to become the Flash. At the end of the season, he was erased from existence.
In season 5 of The Flash, Reverse Flash is seen in a cage in the future, here he is in the form of Harrison Wells.
How is that possible?