Nightmare Fuel: This film is considered a partial horror film for a reason. Smart camera techniques, timing, and sound design managed to make a man in a cheap gorilla costume terrifying.
Special Effects Failure: The monster costume for the movie is nothing more than a cheap Halloween gorilla suit. That said however, clever film making techniques made the costume very hard to see in all but a select few "blink and you'll miss it" moments. Most of the time the suit actor was kept out of focus so you only saw his outline, giving him an eerie quality.
Tear Jerker: The monster itself was even documented and stated as to have never done anything malicious except for self-defense and necessity, and even after he was attacked for a time or two, he disappeared into (self-exiled) solitude before returning and becoming more violent. Even the song on the title page says how the creature might've wondered why he is/was alone, and it's not that hard to feel at least SOME sympathy, if not empathy, for him/it.