Johto
Heracross first appeared in A Sappy Ending. In the 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 the Pinsir were able to return to their forest, leaving the Butterfree and Heracross. However, the same Heracross was still intrigued by Ash and followed him for a little ways out of the forest. Eventually, Ash saw it following and the Heracross happily allowed itself to be captured.
Heracross proved to be a powerful battler in Roll On, Pokémon! where it battled a Donphan and against a Scizor in Wired for Battle!. It also likes to suck sap from plants or trees, as well as Bulbasaur's bulb whenever the two were together, annoying Bulbasaur enough to use Vine Whip on him.
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 the 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 into 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 immediately went to eat the sap on Spenser's Venusaur. After putting up a good battle, it lost to Venusaur's SolarBeam.
Sinnoh League
Heracross reappeared in DP182, where it was seen sucking sap out of Bulbasaur's bulb and also sucking on Professor Oak's head, making him angry. It was used for Ash's battle with Nando in DP183. It was put up against Nando's Kricketune, which used Sing to put Heracross asleep. However, while asleep, it was able to dodge Kricketune's Fury Cutter attack, because of the newly learned Sleep Talk attack, and counter it with an also newly learned Focus Punch. After it woke up, it had a fierce battle with Kricketune, which Heracross ended with a powerful Megahorn/Horn Attack combo. After the battle, Heracross showed its affection towards Ash by "sucking on" his head.
In DP189, it was the first Pokémon to battle Takuto in the semi-finals of the Suzuran Conference. It was revealed to have learned Hyper Beam, which collided with Darkrai's Ice Beam. It tried to attack Darkrai with Megahorn but was put to sleep by Dark Void. Due to Sleep Talk, it was able to hit the legendary Pokémon with a Megahorn, only to be defeated when Darkrai uses Dream Eater to attack Heracross. It became Ash's first Pokémon to fall in his semi-final battle against Takuto.