Even with it's Pre-Evo available earlier and the fact it's Pre-Evo exists at all, I have some reasons to believe Ulgamoth is a Legendary.
First off all, Ulgamoth is 4td strongest non-legendary and non-psuedo-legendary, and the strongest of such in the Isshu dex without a hindering ability like Archeos. Secondly, even though past Pokemon have been an Only One encounter in the past, Ulgamoth is not just found on some random route or cave, as it's found in an ancient castle (Rotom is found in the old chateau, but that sounds more like a random place due to how modern the building is in comparison to Ancient Castle). The most unusual fact about this encounter with Ulgamoth is it's high Level (At Level 70) and having the 2nd lowest non-legendary catch rate at a catch rate of 15 (Only Beldum's line goes lower). It also has the honr of being the highest leveled Pokemon on Adeku's team (Which can't be used as evidence against it being a legendary due to past gens having trainers with legends as well, such as Brandon and as recently as N).
Now I might be crazy, but remember, there was a big debat over Phione for similar reasons yet is a Legendary due to the fact it can't be entered in the Battle Frontier and Official Tournaments. If Ulgamoth is banned from the Battle Subway and Official Tournaments, we may very well have another Phione incident among us. Any thoughts?