I have seen this short on a few occasions on TV (10-15 years ago on). The score by Grieg - In the Hall of the Mountain King was always played with this. It may or may not be an official short for this score, but I cannot locate it anywhere. The short film is about a little boy walking into a cave. The style of the animation reminds me of Van Gogh - A Starry Night. As the music builds up to the climax you see the little boy and some creatures dancing around a fire in a circle... faster as the music tempo increases. Finally, it zooms out and you see the devil high up looking down at them as he laughs. A long shot I know, but does anyone know the artist of this short, or where I could find it please? Thanks!

I'm pretty sure that's a scene in Disney's "Fantasia"

The scene from Fantasia does feature a massively looming Devil, but doesn't have the other stuff mentioned in the query. (I just watched the YouTube clip.) It is set to classical music, but in this case it's "Night on Bald Mountain", and not "In the Hall of the Mountain King".

So - I found something that might fit, a company called Mel-O-Toons used to do animated shorts in the 60's - they have one titled 'Peer Gynt in The Hall of the Mountain King' - have no idea if this is what you're looking for - I am unable to find a clip of it to verify, but here is the wikipedia page referencing it - hope it gives you enough info to continue your search!
Mel-O-Toons - Wikipedia, the free encyclopedia