How to use mobs to make players "turn into" those mobs, but teleport the mob somewhere else if they change

Alright so for a pvp map, im making a Druid class and they're gonna use polar bears and ocelots with custom names for "forms". I was wondering if I could make it so that I could teleport the polar bear to them if they hold an item, but if they let go, then the polar bear is moved away off of them.

As far as I am concerned, There is no way to do so in vanilla minecraft, but to make the player turn into a mob, you could either do one of two things:

• Make the player invisible and then summon the mob being "mounted"
• Or to start a command chain to teleport the mob to the players coordinates constanly