Tarken and Rorak are brother, both are werewolfdog, in other words, they have wolf and dog blood in their DNA.
But with different percentage being:
Tarken 60% wolf and 40 % dog
Rorak 40% wolf and 60% dog.
That is why Rorak is smaller and much less muscular than his older brother Tarken despite being from the same family.
Tarken is 30, rorak is 27.
Strangely Tarken is less aggressive than his younger brother, he acts more like a dog than a wolf, he generally doesn't get into as much trouble as Rorak does.
Rorak, despite being more dog-like, is more aggressive, acting more like a wolf than a dog.