I'm mostly into dragons and slimes, possibly demons.
Since dragons are pretty greedy in general (both in terms of food or gold), I feel like you'd have to reward them pretty favorably (or them) for whatever they might supply you with. Obviously it'd have to be worth it, so I imagine that they would give you some valueable items in turn - dragon eggs, scales, flame-something-liquid, magic powers, defense against outside attackers, knowledge about the location of certain items (missions?), etc.
Slimes would give you... well, more slime? You easily could give the slime output a bit of purpose (other than as food) tho. Could be used as fertilizer, stomach rub or as oil for machines.
Demons idk about, they're just sexy lol.