Question: can zacian be caught in shield?
Answer: Your first foe here will be the opposite game's legendary Pokémon, so Zamazenta(fighting/steel)if you purchased Sword, and Zacian (fairy/steel) if you purchased Shield. Try as you might, this Pokémon cannot be captured. They only exist as an obstacle for your version's legendary Pokémon, so feel free to defeat them.