TOS has a very definite Aesthetic sense, and Robby might not fit as smoothly as Nomad does, but the suit wouldn't be as artistically jarring, as if, say, there were a 1970-1980s style weather beaten Y-wing fighter with faded paint sitting in the shuttlecraft bay, dripping grease and oil on the nice clean Starfleet deck.
But, as others have pointed out, budgetary costs were a far more major factor in the show than we the audience would have liked. ("
The Mark of Gideon" set almost entirely on the Enterprise set with virtually no extras just
Screams like a cheap cost saving measure.) Third season just got cheaper and cheaper and cheaper.
But, Robby could have been an alien robot from a different civilization and technology. After all, it's not like the 1930s Fliver Kirk drives in "A piece of the action" fits the usual TOS aesthetic, any more than the windsock dipped in concrete we all know and love as the Doomsday Machine.
Robby was a master of disguise. He donned numerous different appearances in his various guest roles, changing his arms or the mechanisms inside his head or even plugging his torso into a stationary computer.
I've seen multiple variations of Robby, but if you're describing the one I
think you are where he appeared in "Columbo" the computer they propped his torso on wasn't stationary. They just replaced his legs with a rolling cabinet. (Kind of like a Dalek)
Oh: The Horta came from "The Outer Limits?"
I thought it was an original costume.
What episode did it come from?