Sounds like the ultimate non-Newtonian fluid to me. Also, since objects gain mass as they approach the speed of light, a better visualization might be a falling

Sounds like the ultimate non-Newtonian fluid to me. Also, since objects gain mass as they approach the speed of light, a better visualization might be a falling through air, where drag is mass and terminal velocity is the speed of light. It is not "slowing" so much as it is making going faster progressively more difficult.