Host

Hosts are immobile skulls. When lowered down, which is their normal state, they are invincible to every source of damage except status effects such as poison, burning, being knocked into a wall by, or ' knockback. Occasionally, they will pop up and shoot three spread shots towards Isaac, revealing a fleshy stem that can be damaged. Attacking a Host while it is lowered will increase the delay before it pops up: if it takes what would be a large amount of damage at once they will grimace.

Hosts deal no contact damage either when closed or open.

Red Host
Red Hosts can replace regular Hosts in any area. Their skull is replaced by a vulnerable red fleshlike material. Unlike normal Hosts, they can still take damage when lowered down, but they will fire 5 blood shots instead of 3.

Hard Host
Hard Hosts were introduced in The Binding of Isaac: Repentance. They behave similarly to regular Hosts, however instead of firing any shots when they lift their skull, they slam down and fire a tremor towards Isaac. They also deal damage on contact.

Mobile Host
Mobile Hosts function the same as the original hosts but now do contact damage and slowly approach Isaac. They also have a more random delay between the times they open up, unlike the immobile Host which opens up instantly if it has a line of sight to Isaac and has been closed for a set amount of time without being shot.

Flesh Mobile Host
Flesh Mobile Hosts are like Mobile Hosts, but can be damaged at any time and will fire 5 blood shots instead of 3.

Floating Host
Floating Hosts bounce around diagonally, occasionally opening up to shoot a triple shot like regular Hosts. Unlike regular Hosts, they do half a heart of contact damage and resemble Mobile Hosts more in that sense.

Mushroom
Mushrooms appear as early as and can also replace Hosts. Mushrooms are unlocked by defeating The Lamb for the first time.

Mushrooms start by hiding inside of their cap. While inside, they take only 25% damage. Occasionally, they pop out for a few seconds, shooting 4 red shots in either the cardinal or diagonal directions. Unlike Hosts, Mushrooms deal contact damage.

Trivia

 * Hosts originated from a multiplayer flash game created by Edmund McMillen called HOST.
 * Hosts are very similar to the Met enemies from the game Mega Man; both are immune to any kind of damage when they are lowered down, and when they pop up, they become vulnerable and shoot three shots.
 * Additionally, Mobile Hosts behave like the Neo Met, a version of the Met that walks.
 * In the game's files, Red Hosts and their walking variations have grimace animations that are impossible to see in-game because they are always vulnerable.
 * When Mom's Bracelet is used on a Host/Mobile Host, the resulting Red Host/Flesh Mobile Host will use its respective grimace animation.
 * The Gate physically resembles a giant Host.
 * Using on a Host/Mobile Host will transform it into a Red Host/Flesh Mobile Host.
 * A Mobile Host's body won't change color when transformed.