Freaks it's a horror film
I mostly describe a horror film as a film that involve elements that are beyond normal, such as super natural. Freaks does have those elements, we all know that the actors were real that there wasn't any special effects involved, but the setting of the film real change the whole view of a person. Comparing to what we see in real life, we don't see as many disable people gathering around or disable people turning on people for mistreating them.
The car crush scene it the most horrify scene of them all. The way the freaks were going after the people, they are crawling through the mud, snarling and bearing weapons preparing to exact their revenge, the look on their faces can really scare a person to death and the music behind it. Comparing to aliens there is no different to it and if you find aliens movies scare why not freaks? They both adapt the same type of elements.