I've made it far in the game and I want to really catch this guy. Is it programmed/ a guaranteed catch?
When you fight Eternatus after it Eternamaxes, it has a 100% catch rate, so you can use a Beast Ball on it and catch it!
Eternatus is a guaranteed catch. I caught my Eternatus with a Pokeball. It is 100% catch rate, so you don't have to worry about not catching it.
Hope I helped!