One kind of Ultra Beast. Witnesses have seen it burn down a forest by expelling gas from its two arms.
– Pokémon Moon Pokédex
Celesteela is given the code name UB-04 Blaster. It’s exclusive to Moon/Ultra Moon, as a counterpart to Kartana. You can get 2 Celesteelas in Moon, to trade for a version exclusive I guess.
Celesteela is tied with Cosmoem as the heaviest Pokemon at 999.9kg (2204.4lbs). It’s also very large, at 9.2m (30’02”).
It appeared in the anime, where it was buried in a mountain for centuries, before Sophocles and friends freed it and allowed it to take off like a rocket.
It’s the only other Steel/Flying Pokemon besides Skarmory. Its stats are pretty even (all but Speed are ~100), except its Speed is lower than its other stats.