The belle cats were granted from the animated evolving item, Silver bells. At the end of the evolution, you were given the chance to open the box for one of three randomized possibilities. Blue belle was the most common, then copper, and then Golden.
Silver bells was in the cash shop so it was easier to obtain whereas most of the rest of the cat population on Gaia came from RIGS and were a lot harder to get due to the wide spectrum of possible item grants.
Golden is my favorite out of the belle cats, but gremalkin is my favorite cat overall on this site. Kemuri is cute too, that one is on my wish list.
But enough cats, we need more birds. sad