Echolocation. The hook horror can’t use its blindsight while deafened.

Keen Hearing. The hook horror has advantage on Wisdom (Perception) checks that rely on hearing.

Slithering Physiology. When the abberous hook horror suffers at least 20 damage in a single attack, one of it's limbs is severed, quickly turning into a pile of mush where it lands. 2 more hooks with which it can make attacks with immediately erupt from where the limb was severed. 
If the abberous hook horror starts its turn with more limbs than it ended its last turn with, it gains 10 hp for each new limb. 

Oozing Body. The abberous hook horrors skin is translucent, and made up of a strange gelatin-like substance. It has advantage on dexterity checks and saving throws, and cannot be grappled or restrained. This trait does not function if the abberous hook horror suffers thunder or acid damage. 

Multiattack. The hook horror makes as many hook attacks as it has hooks to attack with. 

Hook. Melee Weapon Attack: +6 to hit, reach 15 ft., one target. Hit: 11 (2d6 + 4) piercing damage.

Many Hooks. The Abberous Hook Horror can make as many opportunity attacks as it has hooks to make them with.