Scolex's main method of attack is to pop out of the ground and jump at Isaac in an arc, and then burrowing back into the ground. He is completely immune to damage, except at the end of his red, blinking tail.
Scolex will also sometimes poke a part of his body out of the ground, exposing either his head or his tail. If it's his head, he will fire 3 IPECAC shots at Isaac and dive back into the ground. If it's his tail, he will fire multiple blood shots with varying sizes, similar to Monstro's attacks.
Scolex is hard to hit, but has the lowest health of all bosses in the game. However, if the player has Wrath of the Lamb installed, Scolex's health will be significantly buffed.
When Scolex pops out of the ground, if Isaac hit his tail, Isaac deal much more damage than when he exposes his tail and fire the blood shots.
When Eternal Scolex's head pops out, it shoots 4 IPECAC bombs that leave creep. Shooting at the invulnerable parts (head and body) will cause all tears to be reflected back.
Interestingly, Scolex's teeth (a sketch from the middle of the page in the artbook) look different.
Scolex is most likely heavily inspired by the Lanmola boss from The Legend of Zelda: A Link to the Past.
Scolex is the name for the head of a parasitic tapeworm.
Scolex's head and tail attacks are swapped in Rebirth.
Should Isaac end up dying on Scolex, the game will freeze. Dying with 1-Up, Ankh, or Dead Cat will also freeze the player's game.
When Scolex fires IPECAC shots, one of the explosions can freeze and can be pushed around. When touching a frozen explosion, the player won't take any damage. This may also happen with a normal bomb during the battle.
When leaving and re-entering the room. The frozen explosion will turn into a troll bomb.
When Isaac use Bob's Rotten Head against him, poison clouds appear randomly around the level, but these are visual only. Once Isaac kill him, leaving the room and entering it again will have an activated bomb where the poison cloud last was.
If Little Gish hits Scolex and slows him and he jumps while slowed, he will not move, landing where he jumped. The same thing happens if Scolex (or Pin) is slowed by Spider's Bite.
In the credits, Scolex's sprite shifts between Pin's sprite and his own if the player has Wrath of the Lamb installed.