When stone walls started moving and attacking, the brute’s true identity was this mysterious life-form, which brings to mind an Ultra Beast.
– Pokémon Ultra Moon Pokédex
Stakataka is one of the new Ultra Beasts introduced in Ultra Sun/Moon. It is only available in Ultra Moon, and is the counterpart to Blacephalon. Its code name is UB Assembly. Its Ultra Sun Pokedex entry says that each “brick” is an individual life form.
Stat-wise, it has really high Defense (the 4th highest, after Shuckle, Mega Steelix and Mega Aggron), as expected of a Rock/Steel Pokemon resembling a castle. It also has pretty good Attack. In exchange, it is very very slow, with only 13 base Speed (it is not the slowest Pokemon; there are 6 Pokemon slower than it).