Fair warning, this is going to be a tl;dr situation...
If you don't mind me getting on my soapbox, I'd just like to say that the 90's had more good horror films than its usually given credit for. Granted, when compared to the sheer mass of films that flooded the 80's then sure, the 90's might have seemed a bit slower but I'd hardly call it a "dry spell" or a "down time" for horror like I've seen it referred to many times. The problem with the 90's lied not with the films but rather with distribution with so many films going direct to video. Even if a film got a theatrical release it was so ridiculously limited it might as well have gone DTV anyway so a lot of films probably lost a good portion of their potential audience, those that didn't spent a lot of time in video stores constantly hunting for something new or read the genre rags, because of that.
American horror in particular seems to get a bad rap and I've always felt that it was mostly because of people thinking back to a post-
Scream (1996) ecology were seemingly every new horror film was a Kevin Williamson penned meta knock-off starring all the hot teeny boopers of the day which is totally understandable but its important to note that prior to
Scream, there were hardly any bandwagons to hop on in horror since the 80's had ended so a lot of the American product from the late 80's/early to mid 90's was pretty original. Now I have no clue what you've seen or not seen so you very well have seen every one of these films but these are some from your requested decades that immediately spring to mind. Also, you said "early 90's" so 1995 was my cut off year:
Gothic (Ken Russell, 1986)
White of the Eye (Donald Cammell, 1987)
The Lair of the White Worm (Ken Russell, 1988)
Deadly Dreams (Kristine Peterson, 1988)
Dr. Caligari (Stephen Sayadian, 1989)
Santa Sangre (Alejandro Jodorowsky, 1989)
The Guardian (William Friedkin, 1990)
Subspecies I-III (Ted Nicolaou, 1991-1993) *The 4th film in the series from 1998 is really good too.
The Borrower (John McNaughton, 1991)
Dust Devil (Richard Stanley, 1992)
Lower Level (Kristine Peterson, 1992) *More of a thriller but there are heavy psych-horror elements to be found.
Body Snatchers (Abel Ferrara, 1993)
Return of the Living Dead 3 (Brian Yuzna, 1993)
Night Terrors (Tobe Hooper, 1993)
In the Mouth of Madness (John Carpenter, 1994)
Castle Freak (Stuart Gordon, 1995)
The Addiction (Abel Ferrara, 1995)
Tales From the Crypt Presents: Demon Knight (Ernest R. Dickerson, 1995)
The Prophecy (Gregory Widen, 1995)
Europe was still delivering quality product in the late 80's going into the 90's thanks to the arrive of new auteurs like Jörg Buttgereit who was causing a ruckus in Germany with
Nekromantik (1987),
Der Todesking (1990),
Nekromantik 2 (1991) and
Schramm (1993).
Italian horror had new life breathed into it with new blood like Gianfranco Giagni's
The Spider Labyrinth (1988) and Michele Soavi's
Stage Fright (1987),
The Church (1989),
The Sect (1991) and
Cemetery Man (1994).
The old Italian guard was still going strong as well with Dario Argento delivering one of his finest films in
Opera (1987), Ruggero Deodato's supremely off the wall killer telephone romp
Dial: Help (1988) and his erotic psych thriller
The Washing Machine (1993). Giallo master Sergio Martino had the berserk action/horror hybrid
American Tiger (1990), one of the oddest films you’ll ever see with its combination of rickshaw pulling, conspiracy, witchcraft and demonic TV preachers. Umberto Lenzi's last horror film to date,
Black Demons (1991) is a really cool take on zombies mixed in with voodoo and possession and Lucio Fulci, who's post-
New York Ripper (1982) output is often put down released the patchwork gorefest
Cat in the Brain (1990) which pre-dated Wes Craven's
New Nightmare (1994), another self-reflexive film featuring a director haunted by his past work by 4 years. Fulci’s final film
Door into Silence (1991) is also an effective low-key little chiller that wound up becoming eerily prophetic given its subject matter.
Spanish horror maestro José Ramón Larraz made three solid films around this time,
Rest in Pieces (1987),
Edge of the Axe (1988) and
Deadly Manor (1990). While tailored for the American video market, all three feature several quirks unique to Euro horror making them stand out in their respective subgenres.
That’s not even mentioning the plethora of Asian stuff that came out around the same time!