The Probotector games are altered versions of Konami's early Contra titles. The series includes Probotector (Contra, NES), Probotector 2 (Super C, NES), Super Probotector (Contra 3, SNES), Probotector (Operation C, Game Boy) and Probotector (Contra Hard Corps, Genesis/Mega Drive). No arcade Contra games were changed.
They were released in PAL markets and included a number of changes, chiefly to tone down the violence for German release (under German law, any video game featuring humans killing humans cannot be displayed on store shelves or advertised and may only be sold to over 18s who specifically request them - the Contra games in their unaltered form would not have sold anywhere near as well in Germany, one of the biggest PAL markets for videogames).
All humans in Contra, including enemies and the main characters Lance and Bill, were replaced with robots for Probotector - the gameplay remained identical, however, as the only changes were graphical or textual.
Konami gave up on these changes with the release of the Playstation's shit-awful Contra games, which made it to Europe unchanged. I am unsure as to whether Germany has relaxed its video game censorship laws, or if Konami just couldn't be bothered changing 3D characters, but the Probotector series ended after the Mega Drive game. The name actually made a brief return for the first volume of Konami GB Collection - in the European version, Contra was renamed Probotector, but the characters were unchanged. So the name didn't really make much sense.
Konami of Europe also announced a new Probotector game for the Gamecube - this was never released, but it is unknown whether they were referring to a Contra title or a revival of the Probotector robots.