There are only 2 options to this. You either didn’t remember what is was properly, or it glitched out.
I did my research, and there are no nature changing glitches as far as I can see. So, you could have glitched your game out in a way that barely anyone has done, or you forgot the original nature of it before and when you saw that it had a rash nature, that was actually the nature it had all along.
If you are absolutely sure it was timid before, then you must’ve glitched your game. It is a GameBoy Advance, not the newest console, so you probably just glitched it.
Hope I helped!