Question: has ash caught a legendary?
Answer: Other than that, Ash did not own any other legendaries. He did own a pseudo legendary, Goodra, in X/Y however. Poipole is an Ultra Beast, Not actually a legendary/Mythical. Also to note is that the Tapus are Land Guardian Pokemon, not Legendaries/Mythical, and Type: Null and Silvally are not Legendary/Mythical.