There is a Lord Shen Petition to bring him back.
I found it and here is the site:
Riolu in Equestria: Team Rocket's InvasionTeam Rocket and their leader Giovanni found a portal to another world. Jessie, James and Meowth are scared for what’s ahead of them. When they go through the portal, their bodies change. After the transformation, they became ponies; however, Meowth stays the same.Riolu in Equestria: Team Rocket's Invasion by gundamfan2
Giovanni and Team Rocket explore this world. Team Rocket asks their leader how he found the portal. He tells them that someone gave sent a package of a map to find the portal.
They saw Earth ponies, the unicorns, the Pegasus ponies, and alicorn ponies. They also see the ponies coexisting with the Pokémon.
Giovanni wants the Pokémon and the ponies to serve him. Giovanni and Team Rocket begin capturing ponies and Pokémon despite being ponies themselves.
Meanwhile, Riolu is forced to wear Rarity dresses and makeup. Riolu is forced to wear an Equestria Girl Rarity white and purple dress. Riolu says, “This skirt is a little too short. Is wearing a girly purple panty briefs necessary? Even the purple lips