Dragon showed up in the first week and took 400 gold. Rejected the blacksmith's idea because I didn't have enough coins. Now that I've completed the game got enough gold I wanna go back and kick its ass but the dragon doesn't show up at all.

Yeah the dragon doesn't come back - but there will be a blacksmith who offers to make the dragonslayer sword. Say yes to that, and then eventually say yes to the knight who wants to slay the dragon. You'll get 500 gold back.