Games like Slay the Horde
Games like Slay the Horde include Halls of Torment, Megabonk, Vampire Survivors, Death Must Die, and BALL x PIT. We matched them by the tags players give Slay the Horde on Steam (Bullet Heaven, Action Roguelike, Roguelite, Bullet Hell, and Pixel Graphics): closest matches first, and among equally close ones the better-known games.

Halls of Torment
Shared: Action Roguelike · Roguelite · Bullet Hell

Megabonk
Shared: Bullet Heaven · Action Roguelike · Roguelite

Vampire Survivors
Shared: Action Roguelike · Roguelite · Bullet Hell

Death Must Die
Shared: Action Roguelike · Roguelite · Bullet Hell

BALL x PIT
Shared: Roguelite · Action · Bullet Hell

Deep Rock Galactic: Survivor
Shared: Action Roguelike · Bullet Heaven · Roguelite

Soulstone Survivors
Shared: Bullet Heaven · Action Roguelike · Roguelite

The Spell Brigade
Shared: Bullet Heaven · Roguelite · Bullet Hell

Boneraiser Minions
Shared: Action Roguelike · Bullet Hell · Roguelite

20 Minutes Till Dawn
Shared: Action Roguelike · Bullet Hell · Roguelite

Enter the Gungeon
Shared: Action Roguelike · Bullet Hell · Pixel Graphics

Necrosmith
Shared: Action Roguelike · Roguelite · Bullet Hell

Rogue: Genesia
Shared: Bullet Heaven · Action Roguelike · Bullet Hell

Tower of Babel: Survivors of Chaos
Shared: Bullet Heaven · Action Roguelike · Roguelite

Nuclear Throne
Shared: Action Roguelike · Pixel Graphics · Bullet Hell

Skul: The Hero Slayer
Shared: Action Roguelike · Action · Pixel Graphics

Nordic Ashes: Survivors of Ragnarok
Shared: Bullet Heaven · Action Roguelike · Bullet Hell

Seraph's Last Stand
Shared: Action Roguelike · Bullet Hell · Pixel Graphics

Magicraft
Shared: Action Roguelike · Roguelite · Bullet Hell

Nomad Survival
Shared: Action Roguelike · Roguelite · Bullet Hell

Spirit Hunters: Infinite Horde
Shared: Bullet Heaven · Action Roguelike · Roguelite

Atomicrops
Shared: Bullet Hell · Action · Action Roguelike

Brotato
Shared: Action Roguelike · Bullet Hell · Bullet Heaven

UnderMine
Shared: Action Roguelike · Roguelite · Pixel Graphics