ok I'm replaying through heartgold and I just beat the 1st gym plus I remember where to catch heracross so I'm want to know what Pokemon can I catch early game that learns headbutt before level 20.
You can get Headbutt by a Move Tutor in Ilex Forest. Although this requires you to have defeated the second gym, Heracross can be found in Alazea Town, near Ilex Forest.
Hope I helped!
