Mike never gave me better than a purple (still, a free purple gun beats a slap in the face), and vendors do RARELY offer a legendary (i have only ever seen one and it was a granade mod in the ammo dump at terramorhous peak), and I have never been lucky enough to get a legendary from a chest anywhere. That said, The chances of her dropping the Nukem are supposed to be about 5% whereas Terramorphis seems closer to 0.01%. It usually drops off of a named spiderant called The Black Queen, found in the Dust if i am not mistaken. The legendary orange weapon i got is called the Derp Nukem. I farmed Terrramorphis for about 14 hours and only got one legendary weapon.
I have seen for myself that any enemy with "the invincable" in their name can drop any legendary in the game BUT the chances of them dropping one are VERY low. This gets expensive REAL fast (it scaled based on level, at level 19, I had to pay $363 a pull, my friend was level 22, he was paying $550+). OH! You can also get Legendaries (citation/confirmation needed, this is 2nd hand) from the Slot Machines in Moxxi's at Sanctuary.
*Right at a set of stairs leading up to the main Sanctuary centerĭidn't know you could get Legendary's from him, best I got was a Purple on my first meet, then all Blues. *Beside the "This Just In" booth (This is where you'll encounter him on your first trip to Sancuary *On the other side of the radio in Moxxi's He's usually in the following locations in Sanctuary: I'm guessing a lot of those are from the side missions or you'd get them by killing BL2's "Secret Final Boss".