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The Protectobots first appeared in "The Revenge of Bruticus", the second episode to feature their recurring nemeses, the Combaticons, with no explanation being given as to where they had come from. They didn't form Defensor until "B.O.T." where they engaged Bruticus on two occasions, achieving victory in the first but nearly suffering defeat on the second. They would continue to appear off and on throughout the remainder of the series and during Transformers: The Headmasters, most often forming Defensor in order to face off with Bruticus.
"Ask Vector Prime" later revealed an origin for the Protectobots not seen in The Transformers; found dormant in space by Omega Supreme, they were reactivated by an Energon infusion from the Aerialbots. This process unexpectedly gave them the ability to merge into Defensor.
Robots in Disguise
"Ask Vector Prime" also introduced a Robots in Disguise version of the Protectobots known as the Protection Team. Sadly, in at least one continuity the team was decimated by Megazarak, with only Hot Spot surviving. After being granted the Matrix of Leadership by Ultra Trion, he became a new Defensor infused with the abilities of his fallen teammates. 
- Defensor (Combined form)