How to make a Lava Entity that swims in Lava

So Im trying to make an entity that swims in tar well lava because tar acts like lava in my mod and it won't spawn in it nor will it swim it will just sit there. I have tried everything and at one point I thought I found a solution only to realize it wasn't a solution it was a crash the game. so if anyone can help me do this that would be great.