We were in Arillas recently, and although we didn't see it for ourselves, some people we met on the beach saw a "fish" in the shallows that had partly buried itself in the sand but had what they described as an "Iguana-like" head sticking out. When we described this to Jimmy Anthis at the Rainbow complex he told us it was a dragon fish, which is very painful if trodden on. He told us of one person who had to come home early because of the pain as there's only so much the doctor can do for you.
Fortunately I think that this is a rare occurrence but it is worth taking note of.
By the way, it didn't stop us from enjoying the sea, we just took as much care as possible.
Steve & Val