Werewolf

A werewolf as ally is a cursed PC's mentor, a reluctant lycanthrope hiding from a hunter's order who needs the party's help, or a wolf-blooded ranger who has made peace with what she is. The framing has to acknowledge the curse openly. The werewolf is not a clean ally. Bite a Humanoid in combat and that Humanoid risks the curse, so the party has to discuss the rules of engagement before they fight side by side.

In combat, run her as a flanker who uses Pack Tactics with the party's frontline. Position the werewolf adjacent to the fighter's target, and both of them attack at Advantage. Open with Longbow at range while closing, then Shape-Shift on the move into hybrid form for Multiattack: Scratch and Scratch, or Scratch and Bite if the target is not Humanoid (an undead, a fiend, a beast). Save Bite for non-Humanoid enemies the party wants dead anyway, since the curse only triggers on Humanoids.

Out of combat she is the most useful Perception and Stealth tracker the party has access to at low levels: Perception +4, Stealth +4, Darkvision 60 ft., and the wolf form's 40 ft. of speed means she can scout a forest perimeter faster than anyone else in the party. The cost is the moon. The party should know which nights she will be unreliable, and the GM should occasionally have her disappear into the trees for a long night with no apology in the morning.

The relationship ends the day a PC or NPC the party loves catches the curse. Have that day eventually arrive.

A werewolf is a horror NPC first and a CR 3 monstrosity second. Open the encounter in humanoid form. The werewolf walks into the inn, sits at the bar, talks to the party in Common, and then either follows them out or watches them leave. Shape-Shift is a Bonus Action, so the moment violence starts the werewolf becomes a Large hybrid or a Medium wolf and the conversation is over. That delay between recognition and transformation is the whole horror beat.

In hybrid form, the werewolf has Multiattack: two attacks split between Scratch (+5, 10 slashing) and Longbow (+4, 11 piercing, 150/600 ft.), and one of those attacks can be replaced with Bite (+5, 12 piercing, plus the curse). Lead with Bite on a Humanoid PC every round it is available, because the curse is the encounter's payload. DC 12 Constitution save on a hit; on a fail the target is cursed, and if they later drop to 0 HP they become a Werewolf under the GM's control with 10 HP. Make sure the table knows that rule before it triggers, because a player whose character becomes a campaign antagonist deserves the foreshadowing.

Pack Tactics is the reason to never run a werewolf alone. With one ally adjacent to the target, the werewolf attacks at Advantage. Add wolves, dire wolves, or other werewolves and the math tilts hard. Two werewolves flanking a fighter are landing nearly every Scratch and Bite. A werewolf with a pack of wolves on a moonlit road is a level-3 party's worst night.

The werewolf retreats when Bloodied with no ally support. Wolf form is 40 ft. of speed, more than the party's average, and the werewolf will break for the woods, shape-shift back to humanoid in cover, and walk back into town the next morning with no visible wounds. The party will not catch her. She will be back, and now she knows their faces.

Decide what kind of werewolf this is before the session. A reluctant cursed villager who hides what she is, or a proud lycanthrope hunting deliberately. The combat is the same. The conversation before and after is completely different, and that conversation is where the horror lives.