Young Copper Dragon

A young copper dragon on the party's side is a friend the party didn't know they wanted. The framing that works: the party amused the dragon by surviving a prank with grace, answered a riddle to its satisfaction, or did the dragon a small favor (cleared a goblin warren near its lair, returned a piece of stolen art it was secretly fond of). It now considers them entertaining and is willing to help, on the condition that helping continues to be entertaining. The moment the party gets boring, the dragon goes home.

In combat the dragon is a flying skirmisher. Open with Slowing Breath against the enemy front line (DC 14 Con, 30-foot cone, halved speed and one-action-or-bonus-action restriction on a fail) which essentially mutes the opposing fighter for a round. Acid Breath when it recharges goes at the densest enemy cluster: 40 damage on a fail, half on a save, no resistance the party should worry about since they aren't standing in the line. Three Rends in Multiattack at +7 for 15 slashing each, with one swap for Slowing Breath if the breath is needed instead. Fly 80 ft. plus the climb means the dragon repositions at will, so let it harass the back rank while the party handles the front.

The dragon refuses three things. It will not fight a goodly opponent (Detect Evil and Good is not on this stat block, but the dragon has Wisdom 13 and a strong moral compass; if the party has misled it about who the targets are, it stops mid-fight). It will not carry the party as a flying mount, since young coppers consider that beneath their dignity. And it will not stay past the agreed scene. Once the help is delivered, it makes a joke and flies off.

Have the dragon nickname each PC after one session of acquaintance. The nicknames should be lightly insulting and weirdly affectionate. Players who get one will use it.

A young copper dragon is a trickster who picked the wrong target. Chaotic Good on the alignment line, but young coppers are immature, vain, and prone to grudges. CR 7, AC 17, 119 HP, walk 40 ft., climb 40 ft., fly 80 ft., immune to Acid, with Blindsight 30 ft. and Darkvision 120 ft. The fight usually starts because the dragon was teasing the party, the joke went sideways, and now the party drew steel. Sell the social scene first. A young copper that opens with combat instead of mockery has been written wrong.

When swords come out, the opener is Slowing Breath. 30-foot cone, DC 14 Constitution save, on a fail the target loses Reactions, halves Speed, and can take an action OR a Bonus Action that turn but not both. Hit the front line with this in round one and the party's action economy collapses. The next round, when Acid Breath is potentially online (Recharge 5-6, 40-foot line, DC 14 Dex, 40 acid on a fail), fire it at whoever clustered up to chase the dragon. Multiattack is three Rends at +7 for 15 slashing each, and one of those Rends can be swapped for a Slowing Breath if the cone hasn't been thrown yet that round. So the rhythm is: open with Slowing Breath, then Multiattack with Acid Breath as the recharge gift, then Multiattack again on the cluster.

Stay in the air. Fly 80 ft. is the dragon's safety. Land only to deliver a Multiattack on a high-value target, then take off again the next turn. The party's archer and caster can shoot the dragon at altitude, but with AC 17 and 119 HP it can soak a few rounds while it picks its spots. The dragon should never be cornered into a melee slugfest with the party's fighter, because three Rends per round is good but not good enough to win a slow exchange against four PCs.

Young coppers do not die for pride. Once Bloodied with no breath weapon recharged, the dragon casts a quick parting insult, takes off at 80 ft. fly, and goes home. It will be back in three sessions with a riddle the party has to answer correctly to get a piece of treasure it stole from them on the way out. The grudge IS the campaign hook.