I think Polo Bear is lovely. Why can't you call her that on her passport?
My share horse's passport name is very pretentious and way too girly for a 17hh gelding. I use Monty as much as possible and when I have to enter using his passport name add a note asking commentators to just call him Monty!
Can't come up with an improvement. Normally there are stacks of suggestions but so far everybody thinks you have already chosen the perfect name for her..
She's rising 6yo. She's off to a friend next week to be ridden away, my friend has just had a new school built so we thought we'd give PB the best possible experience. She's built like a little tank and is like a mini irish draft. Love her to bits.