Dragons...
Ah the ultimate monster in some games are they not?

I have never really thought seriously about a dragon for a Hero Quest dungeon. Maybe in mirth a few times but in all seriousness ....
It all depends on the dragon.
Heroes of the Hero Quest type would be able to fight a smaller dragon perhaps if they had all equipment upgrades and a number of mercenaries for each hero. But a full blow critter like Smaug. Not gonna happen.
My players try and negotiate with Dragons first. No one wants to die here. The dragon wants to live, the heroes want to live, so it is not the best idea to fight it out.
But we do not parlay with monsters in Hero Quest so it is going to be a fight.
Dragon, Small
This beast takes up 2 squares of space.
Attack 5
Defense 5 Defends on all shields! (Dragon hides are tough stuff and they are enchanted beings)
Body 8
Mind 4
May take a fire attack 3 times per quest. Fills room or hallway with fire. All heroes and monsters in the room or downrange in a hallway take 4 dice damage that is only defended against with Black Shields.
Dragon, Medium
This best takes up three squares long and 2 wide!
Attack 7
Defense 6 Defends on all shields.
Body 10
Mind 6
May take a fire attack 3 times per quest. Fills room or hallway with fire. All heroes and monsters in the room or downrange in a hallway take 5 dice damage that is only defended against with Black Shields.
This Dragon may cast a single spell per quest. This spell is chosen before game of course and it is chosen from the Chaos Spell Deck.
Dragon, Large
This beast takes up 4 squares long and 2 squares wide.
Attack 9
Defend 8 Defends on all shields.
Body 12
Mind 8
May breath fire 3 times per quest. Fire will fill all hallways and rooms with open doors in the dungeon. All possible targets sustain 6 dice damage that is only defended against with Black Shields.
This Dragon may cast 3 Chaos Spells per game. The spells are chosen before play is started.
Potion of Dragon Breath Resistance:
This potion will fortify the imbiber against dragons fire to some extant. After quaffing the potion the drinker will defend against dragon fire with White and Black Shields. This effect lasts until the end of the quest.
Sword of Dragon Slaying:
This Long Sword may attack a Dragon 2 times in a turn. Furthermore the Dragon may only defend against the swords strikes with Black Shields.
Shield of Dragons Scales:
The bearer of this shield may add 1 dice to their defense and when under attack by dragon fire the first two skulls rolled by the Dragons fire are ignored before the defend dice is even rolled. If in a hallway or in direct line of fire of the dragons breath one hero directly behind the shield wielder on the adjacent may ignore the first Skull rolled by the Dragons fire.
Dragons Scale Armor:
This suit of armor provides +2 defense dice and is light enough that it provides no movement penalty. Further this armor provides partial immunity to Dragons fire in that the after fire damage is calculated against the wearer, the damage sustained is cut in half. Combined with the Shield of Dragons Scales this armor set is a dragon fighters dream.
Dragons Slayer Complete Set Bonus:
If a single wielder owns the Dragon Slaying Sword, The Dragon Scale Shield and Dragon Scale Armor then the wielder is completely immune to dragons fire.
So it is said that if you know your enemies and know yourself, you can win a hundred battles without a single loss.
If you only know yourself, but not your opponent, you may win or may lose.
If you know neither yourself nor your enemy, you will always endanger yourself.
~Sun Tsu The art of War~