As revealed in Racing Days!, Mars met Rhyhorn at a Rhyhorn racetrack, where the head coach had given up on training him as a racer. Seeing that Rhyhorn really wanted to become a racer, Mars decided to train him, despite having no prior experience with Rhyhorn racing. Together, the two began to win several coveted Rhyhorn racing trophies and garnered quite a name for themselves.
Rhydon first appeared as a Rhyhorn in The Kalos Region Rescue! (Part 1), where it was ridden by Mars to Lumiose City. In The Lumiose Rampage!, Rhyhorn was used to try and calm down a wild Tyranitar. Despite its best efforts, Rhyhorn was defeated by Hyper Beam.
Rhyhorn was used during the second round of the Battle Chateau Tournament in Fierce Fighting at the Chateau! He battled a Trainer's Glalie and won. He also participated in the Battle Chateau Marsh Class Tournament, where he battled Serena's Skiddo. Despite being at a severe type disadvantage, Rhyhorn was able to emerge victorious.
In A Race for Nostalgia!, Mars, learning that the racetrack where he and Rhyhorn met was to be demolished, decided to challenge the contractors to a Rhyhorn race to save the racetrack. During the race, the contractors attempted to cheat by blocking the path with Rock Slide. However, Rhyhorn managed to smash through the rocks, evolving into Rhydon in the process and winning the race. The contractors, refusing to accept defeat, tried to go back on the deal, but Rhydon managed to defeat their Pokémon with its new Mega Punch. With this, the contractors decided to abandon their plans of demolishing the racetrack. At the end of the episode, Rhydon and the rest of the group boarded a ferry bound for the Unova region.
Rhydon remained on Mars' team when he decided to go to Alola.
Rhydon, as a Rhyhorn, was shown to be a very serious individual, rarely showing emotion. In the past, it seemed to be more expressive, as he was seen to be visibly upset when being told he had no chance of becoming a Rhyhorn racer. It would appear that the training it underwent matured it greatly.
After evolving into Rhydon, he seemed to display a softer side to his personality. This was seen in The Lost Cubone!, where he quickly befriended a young Cubone and often acted as an elder-brother figure. His bond with Cubone was shown to be very strong, as whenever Cubone would get upset, Rhydon would be the one to calm him down.