Vainqueur The Dragon might fit your bill. Hapless thief accidentally reveals the nature of the class-and-level system to a dragon.
The dragon, being a dragon, is absolutely pissed that adventurers are being paid tons of gold to do what he already does for free; that is, save princesses and kill lots and lots of monsters. Of course, dragons "save" princesses so they can keep them for themselves (showing off your collection of live princess is quite the status symbol among dragonkind), and the mass-killing of monsters is basically just a dragon eating his lunch.
This is literally the plot of Vainqueur The Dragon
Assuming you don't mean human reskins (green/white/black/red, with/without ears/wings/horns).
Vainquer the dragon is an obvious mention.
https://smile.amazon.co.uk/dp/B081VRFXBM
The MC is not human, as you may be able to glean from the title if you have sufficient skill.
https://smile.amazon.com/Chronicles-Royal-Pet-Princess-Ooze-ebook/dp/B01N13Q2BD is another of this rare set of non-human, non-iseakied human protags.
This one even rarer as they start out without any ability to think.
If you are willing to accept co-MC, Donut, from Dungeon Crawler Carl, good Donut Bad Carl. Donut is the best cat.
https://www.royalroad.com/fiction/43638/dragons-dilemma Sort of not, but yes.
Dragon turns human, and everything gets a whole pile more complicated, especially when he gets his preferred form from 'reference books' carried by adventurers coming to kill him.
Vainqueur the Dragon was very funny imo
https://www.amazon.com/Vainqueur-Dragon-Maxime-J-Durand-ebook/dp/B081VRFXBM
Try Vainqueur the Dragon. It's an almost satirical take of the litRPG/Isekai genre. I've copied the blurb below:
>Vainqueur Knightsbane is your average dragon: a giant, fire-breathing lizard who loves to take naps on his golden hoard, kidnap princesses for fun, and make the life of adventurers miserable. Vainqueur's only pleasure in life is to watch his treasure get bigger, one coin at a time.
>
>So when a would-be thief turned unwilling minion tells him about "classes," "levels," and "quests," Vainqueur wonders if maybe, just maybe, he should consider a career change.
>
>After all, why bother hunting monsters for free when you can get paid for it?
It is HILARIOUS! The 'would-be thief turned unwilling minion' eventually starts liking the dragon then goes, "OMG, I have Stalkholmes Syndrom!" Read it, and find out why the rich always win!
https://www.amazon.com/Vainqueur-Dragon-Maxime-J-Durand-ebook/dp/B081VRFXBM
Vainqueur the Dragon isn't exclusively a web serial. It's on amazon and the sequel is an amazon exclusive https://www.amazon.ca/Vainqueur-Dragon-Maxime-J-Durand-ebook/dp/B081VRFXBM