It wasn't an actual horror movie, or at least, not in the modern sense of the term, with loads of gratuitous gore. It was actually more of a thriller, although it may have been billed as a horror movie when it came out, back in 1960. ;) But okay, since you and I are the only two members online at the moment, I'll give you the answer: the movie is "
Psycho", by Alfred Hitchcock. It's a classic. ;)
And here's the clue... Near the end of the clip above with the cat, you can hear a woman's voice say
"Mrs. Bates?", and then about five seconds later she screams. Mrs. Bates was the deceased mother of motel owner Norman Bates, and the woman screams because the "Mrs. Bates" she sees sitting in the chair is actually the decomposed corpse of the real Mrs. Bates.
I'm not sure anymore how she died — i.e. whether she died of natural causes or whether Norman killed her — but throughout the movie, you regularly see her shadow in front of the window, and you hear her speaking to Norman in an authoritarian and possessive tone, but all of that was Norman himself, suffering from multiple-personality disorder. And on the rare occasions that you see a glimpse of Mrs. Bates moving the curtain aside in order to look outside, that too was Norman, wearing his mother's clothes and a wig.
It is only when the young woman finds Mrs. Bates' room and goes inside that the viewer discovers that the real Mrs. Bates has been dead for ages already, and that it is Norman himself who is spying on the girls through the various peepholes in the walls, and who killed the girl in the shower. ;)