Are legendaries only available after the story?

I really like landorus for example, i see on the wiki that he is located in the rezzai desert, but he was listed as level 100, so do I have to beat elite 4 first to be able to catch legendaries?

Not all of them. Giratina, Thundurus, and Mew are all available pre e4.

aw, i’ll only have my favourite mons in the post game then :frowning: