Considering LB reveals that Tash is real but (unlike, say, the ancient Greek deities and most of the Hindu ones) is not exactly humanlike — from the descriptions in the book, he's much bigger than a man, has a vulture's head with a cruel, curved beak, and four arms with claws for fingernails, and gives off a foul smell like something dead... ummm... well... let's just say I can't imagine very many human women — not even members of the Calormene royal family — lining up to have his babies.  

So I would say it's almost certainly just a myth conjured up to boost the status of the Calormene rulers and their relatives. Unless Tash can take other, somewhat pleasanter forms at will. But we've got no indication anywhere that he can (unlike Aslan, who is definitely able to appear in different forms even within Narnia, let alone in other worlds such as our own), so I'll stick with my conclusion.