Ghast

Ghast is the Nether's floating artillery: an enormous, slow, jellyfish-shaped mob that drifts over open lava seas and lobs explosive fireballs at anything it notices from a long way off. You almost always hear one before you see it, because the cry carries much further than the distance at which it can actually pick you out.

The fireball is also the weakness. Any melee swing, arrow or thrown item that connects with one in flight sends it back the way it came, and a deflected fireball striking the Ghast that fired it kills the mob outright — no gear required, which is what makes an early Nether trip survivable at all. Impacts start fires and blow apart weak blocks, so a nether brick or blackstone walkway survives what a wooden one will not.

Ghast tears are the reason to bother killing one. Nothing else in the game produces them, they are the base of Potions of Regeneration, and End Crystals need them too. Ghasts also need a large open volume to appear in, so keeping them out of a base is a matter of geometry rather than torches: roof your tunnels and build tight, and a ghast farm works by doing the opposite — offering one big legal spawning space and nowhere else.

A Ghast cannot be tamed, ridden or fed; the friendly version is a different mob. A dried ghast placed in water rehydrates into a ghastling, which grows into a happy ghast, and fitting that with a harness rather than a saddle turns it into a flying mount several players can share. Snowballs are what a happy ghast follows and what speeds a ghastling along — the hostile Ghast eats nothing at all.