When Platinum set off on her journey to Mt. Coronet, she took Ponyta from her mansion with her.
Rapidash first appeared as a Ponyta in Stagestruck Starly, where he was used as Platinum's steed. He was mainly used as her mode of transportation, not seen in many battles until A Conk on Cranidos's Cranium, where he participated in Platinum's first Gym battle against Roark and his Onix. The battle ended quickly, however, as he was defeated by a combination of Double-Edge and Slam that sent him flying into a wall and was knocked out as a result.
In Ring Around the Roserade II, Ponyta participated in Platinum's Double Battle alongside Prinplup against Gardenia and her Roserade and Cherubi. Despite the type advantage he was quickly defeated due to Roserade's poison thorns, leaving Prinplup to battle by herself.
In Mirages of Mismagius I, Ponyta participated in Platinum's Gym battle against Fantina and her Mismagius. Due to Mismagius's illusions Ponyta began to panic and was taken out by a single Dark Pulse.
In Brash Bronzong I, Ponyta battled Byron and his Bronzong. Fueled by his Trainer's determination, he defeated both Bronzong with one Flamethrower. In Hurrah for Rapidash after refusing her father's plea to not do anything dangerous, Ponyta evolved into a Rapidash after Platinum expressed her resolve to defeat Team Galactic.
In To and Fro Froslass, Rapidash participated in Platinum's Gym battle against Candice where he battled her Froslass. Rapidash initially held his own due to his fast speed, but was recalled after suffering several hits and replaced with Lopunny. After Lopunny sacrificed herself with Healing Wish, Rapidash returned to battle, fully healed, allowing him to defeat Candice's Froslass, earning Platinum her seventh Gym Badge.
During the battle against Jupiter in Stopping Sableye, he helped alongside Platinum's other Pokémon as well as Candice and Maylene's. Despite their best efforts, the trio were unable to stop Jupiter from capturing Uxie and were defeated, After Jupiter left, Platinum took the unconscious Candice and Maylene to the Snowpoint Temple to rest. Platinum had Rapidash use his flames to keep them warm while they were trapped in a blizzard.
In Shorting Out Electivire, Rapidash was Platinum's last choice in her battle against Volkner and called out against Electivire. Rapidash managed to survive a powerful Thunder Punch, proving to Volkner that Platinum had a strong resolve.
In Getting the Drop on Gallade II, Rapidash battled in Platinum's Battle Castle challenge and fought against Darach and his Gallade. Platinum attempted to get Rapidash to defend by using Protect, but he was defeated when Gallade used Feint to bypass it.
In Exit Empoleon, Rapidash fought in Platinum's Battle Arcade challenge. During the battle against Dahlia, Platinum and Dahlia had to switch Pokémon due to the roulette, meaning Rapidash and Platinum's other Pokémon were forced to fight against their Trainer. Although both sides were reluctant at first, both Platinum and her Pokémon manage to fight at their full strength, which eventually ended with Platinum as the victor.
When Platinum went to challenge the Battle Factory in Softening Up Kakuna, she sent Rapidash and her other Pokémon to search for Looker on Stark Mountain, who had gone missing. They soon return, heavily injured and exhausted with a similarly injured Looker and Buck.
In The Final Dimensional Duel IX, Rapidash appeared once more during the battle against Charon inside the Distortion World. During the battle it was revealed that Platinum's whole party had caught Pokérus through her three new Pokémon, powering them up. In The Final Dimensional Duel XI, Rapidash tried to stop Charon from closing the portal to the Distortion World with his machine. Charon quickly uses the machine to control Rapidash and have him attack his own Trainer instead. Rotom then shows up in a Proteam Omega toy and destroys the machine.