User blog comment:Lord Loss/Monster Appreciation Day: Akantor/@comment-4640052-20150911223214/@comment-5569775-20150919005158

Normally, they'd float. For a creature like Akantor to submerge itself in lava it would have to meet 2 major requirements. 1) It needs to be heavy enough to sink, which looking at Akantor, it passes this one. 2) It would need to have thick &/or extremely heat resistant skin that protects the inside of it's body or rather the water that makes up the majority of the inside(assuming water based life forms), from the heat of the lava and is durable to not melt or burn off.

The reason for the second is that how animals/people IRL react when thrown into lava is they skit around on the surface as all the water in their bodies boils out. The steam created by this actually keeps them afloat above the lava for the process; after all the water boils out it will sort of just sit on top and burn. It's a really unpleasant sight and I don't blame movies for not being accurate about it.