>>55865232It wasn't necessarily Guts but rather the anime's interpretation of Guts. Abilities didn't really became a major factor in the anime until Gen 4. Ash's Swellow was established in the same episode he caught it as a Taillow to be really strong and resilient against Electric attacks. Thunder Armor just took that to its conclusion by having Pikachu and Swellow absorb the electricity into their bodies to power themselves up.
The actual ability Guts debuted in the anime with Paul's Ursaring and worked exactly as it does in the games.
