Auto Counter is an ability introduced in Fossil Fighters that returns in Fossil Fighters: Champions. Vivosaurs with Auto Counter typically have, in their models, some sort of indicator that the ability is present, such as armor, spines or spikes.
In the original game, Auto Counter returns 1/10th of the damage received back to the attacking Vivosaur, rounded down. The ability does not activate if the Vivosaur is defeated by the attack nor if it is hit with a special attack where physical contact does not occur.
In Fossil Fighters: Champions, Auto Counter was buffed to return 3/10th of the damage received, rounded down. If a Vivosaur with Auto Counter is hit with a physical attack but is rotated, Auto Counter does not trigger.