Legendary items can be dropped by basically everything that's killable, lootable or that has a red border when moving your cursor over it.
Treasure goblin hunting is actually a popular method of farming legendary items alongside with chests and bosses.
As of now there are no 100% reliable statistics of what drop the most legendary items but I asked a friend of mine with paragon 370 (so I kinda think he knows some things when it comes to farming) and he told me that the most legendaries come from things in this order.
- Killing a boss for the first time (100%)
- Small chests
- Goblins (any type) = Big chests
- Lootable corpses/Destroyable objects (with red borders)
- Regular monsters = Killing a boss for the nth time
- Elite packs (yellow and blue) = Elite bosses
I myself haven't played as much as my friend did but i can say that I see a very similar drop rate when I'm playing.