I found one in my items when I came on. I know it's nothing to do with a lizard event because it's a special. Is it a prize for being TP'd or some kind of glitch?

If I'm not mistaken, the alien plushies are prizes for guessing the right location in the Stargazer game. Have you played that yesterday?

Thanks. I did play it yesterday. There was no event to say that I'd guessed correctly so I got confused.