Curiosity101
Is now at University! :D
Its because of what is to come in a sense.
Flourescent danios weren't made for public sale. They were made for scientific testing. It was after that where people had the idea to sell them. It's also not a mutation, it's a modification. You literally grab the dna for the flourescence and the pop it in a piece of DNA where you know what the dna does and as such you cause no issue to the fish at all.
In that way maybe you can see how it'll never happen to dogs and cats... particularly as they aren't used often (at all) as model organisms. We might see a glowing mouse... but that's about it. And again... it wouldn't necessarily end up for sale to the public. More likely it would stay in labs.