Originally Posted by
z3rK
Mostly this, however not exactly true.
Forsaken, as their name suggests are scourge free from their masters grip, what happened back in W3TFT, when the lich king was under the attack by the Illidan forces and was loosing his might and control over his own undead minions.
Sylwanas, as a "higher level" undead brought back to life by Arthas (avatar of the Lich King himself) was one of the first undead to regain her free will.
However, the Scourge in Loarderon, were not only humans at that time. The Plague was meant to turn humans into mindless zombies, yes, but many of the scourge minions were also raised from the high elves when the siege of Silvermoon took place. There was even a quote on one mission, if I remember correctly, that says "How can we stop them? for every one we kill 10 of OURS fallen warriors raise in their place" or something like that, said to the Sylvanas when they were defending their city.
So, nowadays Forsaken are, imho a composition of human/elves origins. However only Lich King was free in "what do i can bring back to life, I wonder" and Arthas as a avatar of the Lich King was also limitless in that matter. With them gone, gone is the power to raise "anything" as an undead who can have a free will.
Sylvanas herself can't raise anything, and she's using for that Valkyries who however can't raise, with free will, anything but humans and Vrykul.
So, considering that FORMER forsaken were of human/elves origin, now the new forsaken can be brought to life only from humans/vrykuls (there is even an quest in MoP, while playing an undead that shows Your "ancestors", only humans/vrykul are shown there, and as an undead who was brought back to life AFTER the events of W3 the frozen throne, it imho, clearly shows that forsaken can only be made now from those "corpse sources")
On a side note, as many stated already, ANYTHING can be brought back to life (anything but worgens i assume), yet those brought back to life won't be a part of forsaken as they won't have their free will, and they will be bound to whomever they would call their master. And most probably, when their master (call it necromancer woo hoo) dies, they are decimated as a power that granted them undeath is gone(or maybe not, but they'll remain mindless either way).