I very, very vaguely remember an animated movie from my childhood. The main character is a boy who somehow ends up in some monster world, where the monsters are (at least to me at young age) scary looking, not funny. It could've even be that the monsters took over the world or the town or something, but at some point there was something going on in the monster world too. There was at least a song towards the end (and songs all the way through I think) about some magic potion that the boy drank that made him.. something. I know, vague as details but I'd really want to know what this was, it's been bugging me for years.

Could it be the princess and the goblin?

Thanks for the help, but that was not what I was looking for. However, I managed to find out the movie all by meself. Turns out it wasn't monsters as such but giants (time messes up the memory real bad sometimes) but the movie I was looking for was in fact The Big Friendly Giant. I'm so glad I finally got it :)