Powers: Shapeshifting. Mastery of bio-mechanical technology (their ships are as much "grown" as built).
Weaknesses: Zygons' shapeshifting abilities can be curtailed by depriving them access to any "body prints" they may have on file in their computer systems. This can be effected in a number of different ways. Either the subjects may be removed from the system (in which case, the pattern will degrade shortly thereafter), the system may be destroyed, or the link between an individual Zygon and his ship may be severed.
Bio: Not much is known about the Zygons other than their ability to shapeshift, and the unusual nature of their technology.
Their shapeshifting abilities, however, are not absolute. They can't just turn into anything they like at will. They must capture a victim and hook them into their technology in or to obtain the victim's "body print".
Notes: In any given comic (or televised) stories, Zygons are rarely pictured as seen at top right. This is largely because they generally appear in disguise.