The Egg Viper is but one of the dozens of combat mecha built and piloted by Dr. Eggman. Like most of his mecha, the Egg Viper is an armature for the Eggmobile which the doctor must interface with the mecha to operate it. It appears as the final boss of Sonic's story in Sonic Adventure.
Eggman pilots the Egg Viper in battle against Sonic within his Final Egg base. The machine - like its namesake - appears serpentile, with a long segmented tail attached to the rear of the body. The nodes on each of the segments are laser emitters that fire white-hot beams of light with considerable accuracy, which just barely miss Sonic as long as he's moving. A more powerful laser cannon is built on the front of the machine, which charges up and fires an explosive blast. Two wheel-like devices - one on either side of the Viper's body - can detach and be deployed like spinning tops. Sonic is capable of jumping on the flat top of these weapons to use them as platforms, provided the spikes on the sides don't skewer him if he misjumps. The Egg Viper's last weapon is a plasma cannon that can only power up when the body opens up to reveal the charging petals. At the same time, the cockpit is left open and vulnerable. When Eggman attempts this attack, the Egg Viper's tail extends and provides Sonic a bridge that he can homing-attack across to reach the cockpit. The plasma cannon takes a while to charge up, so Eggman would likely only be able to beat Sonic with this if he fired it from a further distance, or at least didn't extend the Viper's tail for Sonic to cross. When the Egg Viper is critically damaged, Eggman attempts a suicide attack and tries to crash on top of Sonic. If Sonic moves out of the way in time, the Viper will crash in the chasm below.
During the Chaos saga from Sonic X (this saga is a direct adaptation of Sonic Adventure's story) Sonic battles the Egg Viper in Eggman's hidden lab in the Mystic Ruins. In the anime, the Egg Viper seems much less dangerous than in the games.