They’re squishy, sucky, wiggly, and have an amazing ability to yell and whisper at the same time. The Zygons were a humanoid octosquid/bacon hybrid that had were venomous, could make a body print (shapeshift) into  someone if they managed to capture the real someone and keep prisoner in one of their science machines.They also lived off of the milk they got from the Loch Ness Monster, which also happened to be a cybernettically enhanced.

 

They were seen in Terror of the Zygons, and Day of the Doctor