Behemoth

Behemoth (ベヒモス, Behimosu) is a hobgoblin; he is the familiar of Amaimon. Whenever Amaimon is not fighting, Behemoth is usually seen with him.

Appearance
Behemoth seems to be a normal hobgoblin or maintains the appearance of one. It always wears a type of leather harness, to which Amaimon attaches the leash or chain he leads it around with. For an odd reason, Behemoth sports a different color scheme than a normal hobgoblin - a dull yellow, cimson and a leafy green. It is also able to transform into a much larger version of itself for combat.

Exorcist Cram School Enrollment arc
Behemoth is constantly seen around Amaimon in different places. Though it is later shown to have fighting capabilities, it does not appear during the fight in Mepphyland.

Exorcist Candidate Live Combat arc
When Amaimon attacks the Exwires' camp, he orders Behemoth to attack. However, Behemoth and Amaimon are blown away by Shura Kirigakure's total shield. When Rin attempts to chase after Amaimon and Shiemi, Behemoth gets in his way. Shura fights Behemoth and arrives later, but it is unknown if it escaped or if it was slain.

Anime-Exclusive arc
Behemoth appears during Amaimon's assault on the Opera Court House. Having grown to immense size, he attacks several Exorcists before being brought down by the combined efforts of Shura and Arthur A. Angel.

Powers and Abilities
Size Alteration: Behemoth, unlike others of the Goblin species, possesses the ability to alter its size, varying from the size of a medium-sized dog to larger than a tank.

Trivia

 * Behemoth takes its name from a massive land animal from the Book of Job, it gained a certain notoriety in medieval art and later works, being included in the Dictionnaire Infernal, a 19th-century guide to demonology.