Well, I had a Froslass.

Now I don't.

Has anybody else had experience with Pokémon randomly disappearing before? I'm 100% positive I had one, that's no question. I've also checked all my games so I know I haven't traded it over to one of them. And if I may have accidentally traded a Froslass to one of you, I... need it back XD. So assuming that's not the case, has the game randomly deleting a Pokémon happened to anyone before? What's your experience with it if it has?

This has happened to me a few times... But one of them was in my silver version when I didn't know there were more than 5 boxes. Maybe it's in the daycare or something? Or maybe just expertly hidden in your PC?

Did you check the Pokewalker? I've sent Pokemon to the Pokewalker and forgotten I've sent them before.
I have no idea what happened if she isn't there ): Hopefully you'll find her!

PokéWalker, thank you! That's very very possible.

—Thanks V-Generate. I found it!