The enemy version of Horror. To fight it you'll have to leave a Radiation Canister untouched in an area, then go on to the next area and do the same with a Big Radiation Canister. Horror will spawn as a mindless enemy breaking out of a canister in the next area, after the one where you left the Big Canister. Defeating it unlocks Horror as a playable character.
Hostile Horror fires a beam similar to playable Horror's active ability. Its body breaks walls while firing the beam. Horror starts with 90 rads that it can use to fire the beam. It tries to pick these rads back up every time and does that from a fairly long distance. It also grabs any other rads laying around. It's possible to steal those rads from Hostile Horror so it can't fire anymore. If Horror runs out of rads it starts to run at the player to deal contact damage.
Horror drops 115 rads, which compensates for skipped Radiation Canisters and results in higher total rads by the price of delayed mutations and the fight itself. Horror uses 90 of those rads to fire its beam, if the fired ones disappear or the player grabs them, Hostile Horror will drop equally less on death. It can't drop less than 25.
If two Radiation Canisters spawn on the same level with the Open Mind mutation, and you leave both unopened, they will count as one Big Radiation Canister. This means Hostile Horror will spawn on the next level.