Wet1, That has actually been done with doggy pooh.
Of course the reality wasn't that the Dog was genetically modified, it was actually down to a particular type of dog food that was on the test market.
Within the tin of shreaded horse oops... dog food

they had mixed into the food small pieces of a illuminous material.
The problems were:
The dog's stomach shouldn't digest the material, because it could make the dog ill.
It didn't glow well enough
The price was higher than normal dog food which meant that the recent years of people using pooper scoopers would be wasting loads of money, just to be puting out in the rubbish the glowy bits.
(I still say they are just normal white or transparent fish with an Ultraviolet Light shining from above. Any fish tank can achieve that look without falling for a scam)