Is this a beast nemesis ?
And after I beat him(and 10 smaller versions of him(he multiplies after death))
The loot chest he protected contained no legendary, just ordinary green and blue loots.
Am I missing something here ? 
Yep, missing that everyday morning prayer to RNGesus it seems
You are in Elite, legendary chance is not that high
^That. And even in Ultimate, Legendaries are not guaranteed from Nemesis chests.
lol You’re level 76. What’re you doing? Go to Ultimate.
The loading tips even tell you that harder difficulties have better drops. You think they were joking?
I thought the legendary was guaranteed from a nemesis
I think I might have heard it somewhere. 
That is definitely not true or we’d be nemesis farming all the time.