Is Ghast a boss?
No, Ghasts are normal hostile mobs that live in the Nether.
They can spawn in basalt deltas, nether wastes, and soul sand valley biomes, so you can find more than one wandering around.
They are definitely not a boss, though can be more tricky than your average zombie. Reason being is because they fly and their attacks hit rather hard. They also contain a unique mechanic which lets you deflect their attacks and shoot them back at them.