I have noticed some peculiar behaviour on the the part of White Tigermen (et. al?): They're timid. There's two xbow traps in my entrance. They cause a few injuries, mostly to mounts and pull the entire siege to a standstill. My enemies don't take another step towards my fortress interior, preferring to just stand there with their feet in the door unless either lured by creatures or slaughtered by marksmen.
Is this... normal?
This is a normal bug/feature of Dwarf Fortress; goblins also exhibit this behavior. In Dwarf Fortress, normal soldiers only follow their squad leader or attack nearby enemies. As a result, when their squad leader dies, they tend to stand around doing nothing. Since the squad leader usually goes first, he gets killed by traps, and then the other soldiers nearby stop moving because they don't have anyone to follow.
Um... So Stranglers and Beak Wolf Hunters..... Are they spos to be that cowardly...
Opened my gates to bait them in, and the Stranglers approached, my archers fired on one maiming it and the rest turn tail and ran, the maimed one getting out of range. The BWHs didn't move an inch. I then told my Archers to approach on foot, they took aim, shot one and maimed it, it ran back to the pack out of range, moved the Archers forward, they let loose at the pack and they turn tail and ran aswell... They even had one mounted on a Giant War Eagle who's name was Stun... The archers ended up cleaning up 2 of the ones they maimed but they were never threatened by them...
Both Stranglers and Beakwolves have FLEEQUICK, which causes them to flee quickly when they are losing a battle. This is because they are early attackers; this works balancewise to make sure they don't pose too much of a challenge if the player can kill a couple, as well as flavorwise, because as wild animals, they are cautious and prefer not to fight prey that can defend itself.
As for why the beakwolves stopped moving, in Dwarf Fortress, when a siege is spawned, if there is no path from the siege squads to the nearest meeting zone or meeting hall, often the squad ai will break and the squads will not move from their starting spot. They may remain still if no meeting zone is presented as an available target, and I have even encountered situations when, after the gates were shut for a prolonged period, siegers failed to budge after a meeting zone target was made available.
In any case, unarmored, weaponless beasts are not really capable of posing a threat to armored, ranged dwarven warriors; nor are they intended to, that being the duty of later enemies.