Some Gen II Pokémon still use recycled Gen IV sprites, leading to occasional stiffness in movement.
HeartGold allows you to explore the Johto region, and after defeating the Elite Four, you unlock the Kanto region. Essentially, you get two full games in one cartridge, offering dozens of hours of gameplay.
The search term "4780 - Pokemon - HeartGold Version (USA)" refers to the specific scene release number for the North American version of Pokémon HeartGold