What drops Dragon's Essence?

Needed for the Dragon Rider PQ pre-quest.

Edit: Got them at Wyverns, and a buddy said she got them at Skeles.
So probably all Leafre mobs drop them (or all Leafre mobs over a certain level).

I just hunted all mine off the cornians (both kinds) in the Forest that Disappeared out of convenience.

how do you get the dragonoir's horn for the mos and thus the flying skill?

You don't need the horn for the skill.
Click on "talk to Tatamo" (the option normally used to buy Seeds) and he will have the option of getting the moss thing you need.