After the Zygon Empire crumbled after their defeat at the hands of the humans the Zygons allied with the Cybermen to defeat the ever growing human empire. The Zygon troops and commanders where converted into cyberform. Cyber Zygons were similar to the Cybermen that originated from humans but had were shorter and faster and most easily recognised by there red head bars. (DW: Cyber)
The Zygon empire began to grow as the Zygons conquered more planets. The Cyber-Zygons managed to gain many emotions back including hatred, anger and the will to not obey. The only units the Zygons were truly loyal to were there own. Therefore the Zyogons turned on the Cyberman and conquered Telos. The Cyber-Zygons were met in battle by the Humans on Telos were they were outnumbered and defeated by the superior army of the British Empire. (DW: Cyber-Zygons)
The Zygons were forced to retreat across their empire by groups such as Torchwood. The Zygons sighted victory after a battle was won on Carron against UNIT but the British army reinforced them and won a counter attack. The Zygons were beaten back to Zygor were they held their ground against the enemy. Nuclear warheads were used to bombard the planet and utterly wipe out the Zygon force. (EDA: The Cyber war).
With that their army was defeated and the Cyber-Zygons were extinct.