I haven’t seen it yet (sorry, I’m 80 years late) but you can get a list of the songs here: en.wikipedia.org/wiki/Fantasia_(1940_film)#Progra…#Program).
I’m not good at music theory, but I’m sure there is a “genre” of classical music that ressemble horror movies (something like the Mozart’s requiem?). Also you should look at the waterphone which is an instrument used to make weird sounds in this kind of movies: www.youtube.com/watch?v=foSJstDFDfg. It’s made of metal, filled with water, and you use a violin bow to play.