The Neon Cave is an area in Adopt Me! located under the main bridge that connects the Adoption Island and the Neighborhood. The respective NPC for this location is Nixie. When talked to, Nixie will elaborate on the purpose of the Neon Cave. She is located to the right of the entrance.
The Neon Cave was created for players to make their pets Neon or Mega Neon. In order to make these special pets, the player needs to place four of the same Full Grown pets (for Neon Pets) or four Luminous (for Mega Neon pets) onto the smaller outer circles. The Neon/Mega Neon version of the pet will then appear in the larger inner circle.