Perhaps, for that reason there are relatively few publishers accepting unagented manuscripts. (Many horror publishers say they are "swamped.")
The publishers I have listed below accept a variety of formats: short stories, poems, and novels. I have not included digital only publishers, or publishers not currently open for submissions. Nor have I included publishers that offer to "crowdfund" books.
Please read all submission guidelines carefully.
Note: You can find a list of over a hundred publishers accepting manuscripts from authors (no agent needed) here: Publishers Accepting Unagented Manuscripts
Arkham House published its first book, The Outsider and Others by master
Submissions: Please submit a synopsis of your manuscript to: PO Box 546, Sauk City, WI 53583. Do not make submissions via email. Arkham House does not return submissions or acknowledge that they have received your manuscript. They will contact you only if they are interested in publishing your work. Snail mail only. Read full guidelines here.
Submissions: Send a synopsis of up to three pages. This should include information about your characters, major plot points, and how the book ends. Also send the first three chapters; a cover letter with your real name, the name you wish to publish under (if different), contact information (address, email), word (not page) count, publication history, and any specific knowledge/experience you have relating to the material. Electronic submissions only. Responds in 1-3 months to queries. Responds in 3-6 months to proposals. Read full guidelines here.
Rainfall Books is a British publisher of fantasy, horror and science fiction, specializing in modern
Submissions: Rainfall has published 150 chapbooks over the past three or four years and have many more waiting in the wings. They are always looking for submissions for chapbooks in the form of poetry and short stories. They are happy to publish reprints. Please read guidelines here.
Samhain Publishing is looking for novels—either supernatural or non-supernatural, contemporary or historical—that are original and compelling. Authors can be previously unpublished or established, agented or un-agented. Content can range from subtle and unsettling to gory and shocking. The writing is what counts.
Submission criteria: Between 12,000 and 120,000 words. For print eligibility, length must be a minimum of 60,000 and a maximum 120,000 words. Books are first published as ebooks, then as trade paperbacks within twelve months. Please read full submission guidelines here.
Tartarus is a small British independent press founded in 1990. They specialize in collectible hardback limited editions of literary supernatural/strange/horror fiction. They also publish paperbacks and ebooks. Tartarus has been the recipient of four World Fantasy Awards, and in 2010 received a "Stoker" from the Horror Writers Association.
Submissions: Tartarus is looking for short story collections and novels of between 75,000 and 120,000 words. They would like to receive literary strange/supernatural fiction. They are not interested in high fantasy, violent horror or young adult fiction. They do not publish teen, children's or young adult fiction. Electronic submissions should be sent to firstname.lastname@example.org as a Word or rtf attachment. Please send a synopsis or first two or three chapters/stories when first getting in contact. Additionally, they are now accepting single short story submissions for Strange Tales V. Please read full guidelines here.