You earned the pet by raiding with them and on top of that everyone choose to give it to you and you still gave the Gbank 50k. In my opion you already went over and above for the guild. I would keep it and enjoy your new pet
What I would NOT do is use it for trade fodder or to sell for a profit for yourself.
"Even when you are winning soundly, you must always give your opponent a way to retreat with honor. If you don't, then he has no reason to surrender. He will fight until the bitter end, and you will pay a larger price for victory."