Trailer Casters?

They are called, "Trailer skid wheels". Do a Google search and you'll get lots of hits. Use to, in days gone by, folks would attach them to the V- shaped skid bars welded under the trailer frame. More and more, you don't see them any more. Maybe they were doing too much damage dragging and digging into asphalt when going over a dip. Attaching the wheels would prevent destroying the asphalt from getting torn up, but the extra length on the skid bars shortens the distance. Not sure how that stress on the rear of the trailer would affect the frame.