how can i make items that catch entities?

You can make a ranged item without munitions (arrow for example), and just create a procedure when one of the Pokeball touch a mob. After, get the entity in a nbt tag (in the item), and despawn the entity. If the pokeball has already something inside, get the entity, spawn it, and remove the text inside.

I’m not sure if it’s work, but you can try this with add some elements because I just say you bases.