From the Super Mario Wiki
Robo Troopas are robots that are modelled after Koopa Troopas. Little is known about them, as they only made a few brief appearances in The Super Mario Bros. Super Show! episode "Robo Koopa". Their voices are identical to those of natural Koopa Troopas. Whether they were built by Dr. Nerdnick or Robo Koopa remains unknown.
Two Robo Troopas were ordered by Robo Koopa to guard Dr. Nerdnick's workshop in Robo Land, where he had forced the poor inventor to stay as his prisoner. When Mario and his friends showed up to rescue Dr. Nerdnick, Luigi accidentally earned the Robo Troopas' attention by getting knocked over. Before the Robo Troopas could even lay a single attack, they were quickly defeated by Mario and Toad.
Later on, a Robo Troopa made another appearance, this time to alert Robo Koopa that Mario and Luigi had returned to fight him in their new Plumbinator suit. After King Koopa was exiled from Robo Land, the Robo Troopas were never seen again. However, similar enemies, such as Mecha-Koopas, have gone on to appear in games like Super Mario World.