HeartGold requires AP patching. Many modern loaders (like TWiLight Menu++) apply these automatically, but older methods may require a manual patch using a tool like Scene Versions:
Any Pokémon in your party can walk behind you in the overworld. 4780+pokemon+heartgold+uxenophobiands+install
Suddenly, the battle music screeched. The screen flashed white. A Wild Pokémon appeared! It was a Sentret. Level 2. Jax sent out REGRET . HeartGold requires AP patching
This specific ROM is frequently cited as the required base for fan-made projects, such as the Moemon Fire Red Revival Project and various "drayano" difficulty hacks. Installation Guide 4780+pokemon+heartgold+uxenophobiands+install