Actually in recent versiins thats not been the case. Sometimes theyll kill each other, too. Ive had this happen myself, where otherwise friendly same-species werebeasts murdered each other.
Dwarf Fortress mechanics are so fascinating. It's possible Toady changed the code because he wanted them to fight each-other, but it's also possible this is an Emergent Property of some other code change he implemented recently.
Mr. Vaga, I believe this means that your Wereskink Macedwarf provides an excellent opportunity for Dwarven Science.
Please report back any results.
Theory: Werebeasts who share the same curse are not hostile, but if they don't they will be hostile - for example, a wereskink cursed by the god Tobul will be hostile to a wereskink cursed by the god Ari, because they don't share the same curse, even if they're the same species of werebeast.
That was my first guess, but
Lozzymandias had 6 migrants who could have been infected by a Werebeast, so he walled them off. After killing the poor schmuck(s) who had not been infected by the werewarthog, the infected werewarthogs started fighting each-other. Two of them teamed up to kill the others. Those two remained peaceful for several months, but after that they fought. There was only one survivor.
Another theory was that it depended on who turned when, but they all seem to turn on the same tick (which is apparently the tick the moon changes to full moon).
So I have no theories.
TheFlame52's loyalty cascade theory makes some sense but does not completely work. A loyalty cascade caused by weretapir a transforming, insta-attacking dorf, who then turns into weretapir b on the next tick, meaning weretapir c is honor-bound to protect weretapir b by attacking weretapir a, etc., would explain it; but they all transform on the same tick.
Perhaps there is some lag in the AI perceiving that loyalties have changed, so they attack each-other, causing a loyalty cascade? OPerhaps Lozzymandias experience was a one-off? Or Saiko Kila's impressive experiments were mistaken and they actually transform on different ticks?
More Science needed.