Wyvern

A wyvern on the party's side is a tamed mount, almost certainly bonded to a single rider through training that takes years and gets handlers killed. Wyverns are not friendly. The common framing is a mercenary order that breeds them or a ranger who saved a hatchling and got lucky. Whatever the setup, write the rider into it. A wyvern without a rider is wandering off to eat the bard.

In combat, the wyvern is the party's air superiority. Fly 80 with a rider in the saddle changes the whole battlefield. Open by climbing to 60 feet, dive on the enemy commander, and let the rider attack with their own weapon while the wyvern Multiattacks. Bite at +7 for 2d8+4 and Sting at +7 for 2d6+4 plus 7d6 poison: that is 50 average damage from the wyvern alone, plus whatever the rider does on the dive. Most tier-2 mooks die to a single pass.

Out of combat, the wyvern is the rider's character. It has opinions about which PCs it likes (the cleric who feeds it, never the warlock who keeps reaching for its tail), refuses to enter cities (most gates do not admit a Large flying creature with a 7d6 poison sting), and roosts on the inn roof while the party drinks below. It does not speak (no listed languages, Int 5), but it is intelligent enough to follow the rider's voice commands and to recognize the party as its packmates.

Have the rider name the wyvern and tell the party one rule about it before the first night in camp (never the tail, never horsemeat). The party that respects the rule keeps the wyvern. The party that does not learns what 7d6 poison feels like at the breakfast table.

A wyvern is the encounter where the party learns what poison can do to a tier-2 character. CR 6, fly 80, 127 HP, AC 14, and a Sting that delivers 11 piercing plus 24 poison (7d6) on a hit, with the Poisoned condition until the start of the wyvern's next turn. There is no save on the poison damage. It is rider-on-hit. A fighter eating one Sting takes 35 average damage and is Poisoned (Disadvantage on attacks and ability checks) when their turn comes around. Two stings in a round drops most tier-2 PCs to single digits.

Open from the air. Walk 30 is irrelevant; this fight is at altitude or it is wrong. Darkvision 120 and Perception +4 means the wyvern sees the campfire from a long way off and dives at night. Round one is Multiattack: Bite at +7 for 2d8+4 piercing (5-foot reach, so it must land or hover within range) and Sting at +7 for 2d6+4 plus 7d6 poison (10-foot reach, so the tail can hit a PC the wyvern flew over). The smart play is the flyby: swoop, sting the lowest AC, climb back to 60 feet.

Target by Constitution save, not by HP. The Sting's Poisoned rider sticks for one round, but the damage applies whether the PC saves or not (there is no save in the action text). Pick the rogue or the wizard. Avoid the dwarf cleric with Constitution save proficiency. The wyvern is a beast (Int 5), but it has been a predator for years, and it knows which prey kicks back.

Wyverns retreat once Bloodied if the prey is fighting too hard. They are solitary hunters, not pack animals, and a 60-HP wyvern that has lost a wing membrane will fly for a cliffside cave and lick its wounds. Have it scream once on the way out. The party should know it is up there, and that it remembers them.