Green Dragon Wyrmling

An allied green dragon wyrmling almost always means the party hatched the egg. Maybe they recovered the clutch from a poacher, maybe a druid's prophecy demanded it, maybe a local lord paid for the unusual pet. The wyrmling imprints in a thin, transactional way: it cooperates while the party feeds it and protects it, and it is already calculating the day it grows large enough to leave on its own terms. Play that long arc openly. The party's good deed is feeding a future villain, and the players should know it.

In combat on the party's side, the wyrmling is air support and a panic option. Poison Breath in a 15-foot cone, DC 11 Constitution, 21 poison on a fail with recharge 5-6, lands well on a clustered enemy mob if the party can angle it without catching themselves. After the breath, it flies to 60 ft. up and uses Rend (+4 to hit, 10 slashing plus 3 poison) on whatever the party tags as priority. It is small and fragile (38 HP, AC 17), so the party's job is to keep enemies from concentrating fire on it.

Out of combat the wyrmling is a problem. It lies. It steals. Amphibious lets it slip away through any pool larger than a bucket. The party will spend roleplay time managing it, and that's the feature, not the bug. If it grows up under their care, it grows up greedy and articulate, and the campaign has its own dragon to deal with later.

Have the wyrmling start naming the party's gear by value. The bard's amulet, the fighter's blade, the price each one would fetch. The party should hear the inventory and feel uneasy.

A green dragon wyrmling is a clever hatchling that has already learned to lie. It is Lawful Evil at age zero with Int 14, and that intelligence is the encounter. Don't run it as a chromatic puppy. Run it as a small, malicious child with claws. Set the scene in a forest clearing or a flooded cavern (30 ft. swim and Amphibious), with a few minor servants it has bullied or charmed into helping: kobolds, scared peasants, a single hag's apprentice trading favors.

Open with Poison Breath if the party is bunched. DC 11 Constitution save against 21 poison in a 15-foot cone, recharge 5-6, and the DC is low enough that one PC will likely take half. After the breath, the wyrmling moves. It has 60 ft. of fly and 30 ft. of swim, so it should never be in melee with more than one PC at a time. Use the trees. Pop up over a branch, two Rend attacks at +4 to hit (10 slashing plus 3 poison each), then climb back into the canopy where the rogue's bow is the only real threat. Blindsight is only 10 ft., so a stealthy PC outside that range still has the usual rules to work with.

The wyrmling has Poison damage immunity and is immune to the Poisoned condition, so a druid leading with Cloudkill wastes the spell. Saves are otherwise weak (+1 to +3), so Hold Person, Banishment, and Hideous Laughter all hit normally. 38 HP and AC 17 means the fight is over in two or three good rounds if the party concentrates fire.

Wyrmlings flee. At 15 HP it bolts straight up, dives into the nearest pool, or hides in the canopy and lies very still. Have it call the party "kind sirs" while it backs away from the cleric. It will be back in five years at twice the size, and it will remember which one of them swung the killing axe.