Johto
It first appeared in A Sappy Ending. In that episode, several Heracross and Pinsir were having a territorial dispute. One of the Heracross followed Ash and his friends as they tried to figure out a solution to stop the dispute, which, unsurprisingly, came in the form of Team Rocket. Team Rocket was stopped thanks to the Heracross, and Pinsir were able to return to their forest, leaving the Butterfree and Heracross. However, the same Heracross was still intriguied by Ash and followed him for a little ways out of the forest. Eventually, Ash saw it following and Heracross happily allowed itself to be captured.
Heracross proved to be a powerful battler. The battles with it versus a Donphan in Roll On, Pokémon! and it against a Scizor in Wired for Battle! show this. It also likes to suck sap from plants or trees. As such it would always try to suck the sap from Bulbasaur's bulb whenever the two were together, annoying Bulbasaur enough to use Vine Whip to make Heracross stop.
Heracross was sent to Professor Oak's lab in Tricks of the Trade in exchange for one of Ash's Tauros. Ash sent Tauros back in the following episode, but Professor Oak informed Ash that he wanted to study Heracross's behavior and wanted it to stay with him for some more time.
In the Johto League Silver Conference, Ash used it in the league battle against Gary. Despite a type disadvantage, it was able to defeat Gary's Magmar but lost to his Blastoise. After Ash returned from Johto, he left Heracross with Professor Oak when he began to his journey to Hoenn.
Kanto Battle Frontier
In Ka Boom with a View!, Ash used Heracross again in the battle against Palace Maven Spenser. Upon being sent out, Heracross automatically went to eat the sap on Spenser's Venusaur. After putting up a good battle, it lost to Venusaur's SolarBeam.