So I was trying to play through Pokémon Blue version and I had a Staryu on my team. It had just reached level 36 and then, it started evolving! I swear I didn’t use a Water Stone on it. Can anyone tell me what’s going on? And also, if it’s worth it to keep cancelling evolution until Hydro-Pump, or just teach it Surf?
In Red and Blue, if a Pokémon that evolves with an evolution stone levels up, and another Pokémon with the same index number as the required evolution stone is on the field when the battle ends, the stone-evolving Pokémon will evolve after the battle. Onix shares its index number with the Water Stone, so winning the battle with Onix after Staryu leveled up caused Staryu to evolve.
