Shamus first appeared in Tepig's flashback in The Battle Club and Tepig's Choice, though much of his features were not revealed. He then appeared in person in Evolution by Fire!, where he was having a Double Battle with Kylan in the Battle Club, using Emboar and Heatmor; Ash and his friends witnessed that battle. Due to his preference for Fire-types, he refers to himself and his team as the "Fire Warriors". He won the Double Battle rather easily. Afterwards, he confronted Kylan and told him to hand his over Pokémon to him, causing Ash and the others to confront him briefly. He ultimately did not take Kylan's Pokémon because of Don George walking by at the time.
Tepig being abandoned by Shamus
While having lunch alone, Ash's Tepig saw Shamus and ran up to him affectionately. It was revealed that Shamus abandoned Tepig after it lost to a Deerling in a battle, despite having the type advantage. Since Tepig kept trying to go with him, he tied it to a post. Seeing Tepig with Ash, he told Ash that he abandoned it because of its uselessness, which struck a nerve with Ash. While Ash and Iris found this to be cruel, he defended his actions and explained that it was done for Tepig's own good, because forcing a "talentless" Pokémon to battle would be worse than releasing it.
To settle the matter, Don George recommended that Shamus and Ash battle each other, and both agreed to be a Double Battle. Ash decided to use Tepig and Snivy, but Tepig did not wish to battle against his former Trainer. He had the impression that Shamus still cared for it and only released it because he cared for it. However, Shamus then revealed that he would only pretend to be pained from releasing a weak Pokémon. This was so that if he ever encountered them again, they would be hesitant to battle him. This revelation left Tepig crushed.
Although Snivy ended up being defeated by a Flare Blitz/Fire Spin combination attack, Tepig managed to persist in the battle and evolve into a Pignite in the process. Using its new attack, Fire Pledge, as well as a powerful Flame Charge, he was able to defeat both of his opponents. Impressed by Pignite's new power, Shamus asked it to come back to him, explaining that it could not forget about him. Pignite simply responded by scorching his head with Flamethrower, before walking over to where Ash was and standing proudly beside him, having decided to stay with him.
This listing is of Shamus's known Pokémon in the anime:
Emboar was first used in a Double Battle with Heatmor against Kylan's Watchog and Mienshao, seen standing up to Watchog's Slam, and heavily damaging the two Pokémon with Hammer Arm. He was used again against Ash's Tepig and Snivy. In the middle of the battle, he ended up being infatuated by Snivy's Attract. This infatuation was short lived as Heatmor's Fury Swipes made direct contact on Emboar, something that surprised Ash as Shamus commanded Heatmor to attack its own teammate. Later, Emboar was able to defeat Snivy using Super Flare Blitz. Although performing many powerful moves, including a combination of his Flare Blitz and Heatmor's Fire Spin, he was eventually knocked out by Ash's newly evolved Pignite.
Heatmor was first used in a Double Battle with Emboar against Kylan's Watchog and Mienshao, seen standing up to Mienshao's Drain Punch, and serving the finishing blow on the two Pokémon with a Flame Burst. It was used again against Ash's Tepig and Snivy. Heatmor defeated Snivy by combining Emboar's Flare Blitz with Fire Spin to create Super Flare Blitz. Although performing many powerful moves, including the aforementioned combination of Emboar's Flare Blitz and its own Fire Spin, it was eventually knocked out by Ash's newly evolved Pignite.