I have a very similar story to yours, Seph. I was playing my Crystal version, When I found a sandslash in the Victory Road. My friend (who I hate now, and no not cause of this, You have to be reallly stupid to hate your friends cause of a video game) told me to catch it with a Master Ball because it was 'Rare'. I believed him and wated it on the Sandslash.
When I was playing Sapphire once, I killed the Kyogre cause I didn't know it was legendary :rofl: