Updating
Trollhound Working Draft.
Assuming you're OK with the Challenge Rating of 2 we are.
Actually, I've just thought of a tweak. What if we added a "mixed pack" of trolls and trollhounds since the two monsters sometimes live together?
We could use the "troupe" organization employed by Lycanthropes:
Organization: Solitary, pair, gang (3–6), troupe (2–8 plus 1–4 trolls) or pack (7–24)
They're capable of tactics, since having a pack leader "
allows them to lead their packs with rudimentary tactics that often catch militiamen and adventurers off guard". I think it's more that they're too ferocious and chaotic to use them.
How about:
A trollhound normally charges into melee as soon as possible and uses its bite attack. Should it achieve a lockbite the trollhound tries to hold on until its enemy is torn to pieces. Trollhounds are intelligent enough to use pack tactics like ambushes and flanking attacks, but are so viciously chaotic they generally only cooperate under the strong leadership of an advanced trollhound or troll beastmaster significantly more powerful than them. A trollhound usually only considers retreat if it is injured and threatened with fire, but even then it often prefers to keep fighting.