Reader-insert
The reader is a skin changer like Beorn.
Only they can change into a dragon; as well as halfway between human and dragon, with wings, fangs, a tail, and retractable claws.

Essentially the reader decides to explore Erebor to find refuge, and meets Smaug. Him being his gold obsessed self isn't too delighted to see a (seemingly) human visitor. He smells them and is intrigued by the reader's scent, seeing as it's a strange, swirling mixture of dragon and human. The reader is slightly frightened by his being there and shifts out of instinct, in order to protect themself. Smaug takes in the scene before him, calculating and thinking to himself. Realizing that you two are likely the only dragons left, and decides you will be his mate, refusing to let you leave. (And it goes from there)

(This is way before the company gets there.)