Ultra-Sentinel's are extremely powerful war-era robots, constructed by Equestria. They are used as guardians for high security facilities, most notably the Ministry of Awesome's hub in Canterlot.
During the War
These massive robots were built as the ultimate security guards, tasked with protecting important locations and facilities during the war. They are linked to a building's security systems, triggering aggressive action against intruders.
After the War
Rainbow Dash arrived in the Canterlot hub of the Ministry of Awesome whilst battling Gilda. She activated the Hub's security systems, including the Hub's robotic guardians, the Ultra-Sentinels which remained inactive until Littlepip and her group's arrival, two hundred years later. The Ultra-Sentinels attacked the group, their prismatic, magical energy cannons proving particularly deadly.
In Other Stories
An Ultra-Sentinel was encountered in the Project Chimera labs, located beneath Hippocratic Research. The Ultra-Sentinel was a deadly opponent that was delayed by a weakened Discord from pursuing Blackjack and then finally stopped by being crushed repeatedly by an elevator.
A heavily modified Ultra-Sentinel is one of the many machines used to guard The Junkyard. How Scraps obtained it and the original condition it was in is currently unknown, so far the modifications seen are an increase in size and the addition of powerful clawed arms easily capable of crushing bone.
Ultra-Sentinels are massive war/security robots, described as almost tank-like. They have a large energy cannon that fires magical, rainbow colored energy beams.