It will enter the third form when it loses enough health and will change its color. During this form it will begin to use thunder magic.
The player should focus on dealing damage, and equip accessories like the Shine Cape, Stun Cape, or Light Shield to make the fight easier, as Behugemoth uses thunder magic, which the Shine Cape and Light Shield resist, and the Stun Cape can prevent getting paralyzed. Having a Black Mage with the Spell Focus ability can help deal damage, preferably Fira, since it can be bought in Urbeth (at night) for only 1500G.
Behemoth is a beast mentioned in Job 40:15–24. In addition to mythological creatures, it is likened to elephants, hippos, rhinos, and bison. Metaphorically, the term behemoth denotes "an extremely large or powerful entity."