Small Dragon (Augmented); CR 5; HD 5d12+10; hp 42; Init +2; Speed 40 ft., fly 100 ft. (average), swim 40 ft.; AC 18, touch 13, flat-footed 16; Attack: bite +9 (1d6+3); Full Attack: bite +9 (1d6+3) and 2 claws +4 (1d4+1); SA Breath Weapon, spell-like abilities; SQ: damage reduction 5/magic, darkvision 120 ft., immunity to acid, sleep, and paralysis, resistance to cold, electricity, and fire 10, water breathing; AL CE; SV Fort +6, Ref +6, Will +4; Str 17, Dex 14, Con 15, Int 14, Wis 11, Cha 12 Feats: Flyby Attack, Hover Skills: Hide +14, Move Silently +10.
Breath Weapon (Su): 20 ft. cone, damage 2d6 acid, DC 13. Once every 1d4 rounds.
Smite Good: 1/day, +5 damage versus good creatures. Spell-like abilities: Darkness 3/day, Desecrate 1/day, Unholy Blight 1/day.
What could be more fun than chasing a hungry, evil, acid-spitting monstrosity through a sewer system? The perfect horror encounter for a low-level party! Give it some appropriately demonic cosmetic features to show its demonic origin (I used three red eyeballs in each eye socket) - but keep these hidden until the last moment so that the horror of the PCs will be much greater when they discover that what they are facing is not "just a dragon"...
In my adventure, I also let the PCs stumble across its egg shells in the place it hatched. The floor was filled with acid - not enough to truly hurt the PC, but it did destroy their boots eventually.