This article discusses the Mega Version of the Neon Pets. The Regular Neons can be found here.
Mega Neons are a special kind of pet in Adopt Me! that was released on April 17, 2020. Players get a Mega Neon pet by combining four Luminous Neon Pets of the same type of pet in the cave under the main bridge. When combining these four Neon Pets, in which they are gifted with a radiant color-changing companion (usually go through the stages of the rainbow). The parts of the pet that are glowing depend on the type of pet the player is turning Mega Neon. The only pets you cannot make mega neon or neon are the Chick, Scoob, and the Pet Rock, which are all from events and are untradeable. There was a twitter post with one of the adopt me team members sitting on a neon pet rock, but that is unobtainable if you are a regular player. The Scoob and Pet Rock also disappeared after the event ended.
The Chick could be hatched out of the 2020 Easter Egg during the ROBLOX Annual 2020 Easter Egg Hunt and can not be traded or gifted, hence why you can not make a neon or mega neon Chick.
Same with the Pet Rock, it was a flyable and rideable pet that was available at the beginning of April 2020, as a fun April Fools event. It was then taken out of your inventory on April 3, 2020. Each user could only have one of these, where they are untradable.
Previously, the Shadow Dragon glowed a turquoise color instead of the usual rainbow glow. However, Adopt Me changed it to rainbow in the Monkey Fairground Update.