I understand that when a creature is under the influence of a polymorph spell they take on the form of the creature they wish to look like and don't change their actual creature type or anything stats beyond what the polymorph spell says you get.
But what I want to know is if it actually changes the name of the form you are taking on. Like if you take on the form of a Hag are you actually a Hag creature despite not being a Monstrous Humanoid?
Well this question was more about being a Hag. What defines a creature as a Hag? It's certainly not creature type so it is by name? What if a player decided to name their character Hag? Does that count? It's not their blood since that's an Accursed Bloodline Sorcerer. Their physique perhaps? Are there other ways to start a Coven without being a Hag from the Bestiary?
Hum, I think one of the problems is that "to join a coven" isn't made explicit in the rules. It's made out to sound like covens are some sort of prearranged agreement: it doesn't seem like three wandering hags can get within 10 feet of each other and spontaneously cast a spell.
I've always interpreted this to mean that hags, witches (w/ coven hex), and accursed sorcerers need to have some sort of formal ritual planned out that establishes their membership in a coven. This way, when their within 10 feet of each other, they "can work together to use any of the" spell-like abilities available to them.
So, in regards to the Monstrous Physique, I am not sure whether simply polymorphing into a hag grants the knowledge of how to form a coven or conduct cooperative magic. Maybe a wizard or alchemist could learn how it's done (by another hag perhaps) and then use the polymorph effect to participate in the coven they've already joined.