The Sentry Robot is an enemy cut from Serious Sam: The First Encounter.
Little is known about the Sentry Robot, as there is no coding for it. The only information that shows it existed is some code for its model in the Serious Sam 1 SDK. According to the SDK information, the Sentry Robot would have three models; Ball, Cannon and Gun. Anything else about it is unknown.
The last edit date for the Sentry Robot's model files is 1/20/2000, over a year before The First Encounter was released. This, along with the fact that it doesn't have an editor icon in Test 1, suggests that the Sentry Robot was cut long before the other cut enemies were.
The Sentry Robot was completely removed from the final game. The only things that suggest it existed at one time is code for its models.