Refusian

Refusians were the original natives of Refusis II. They welcomed the humans and Monoids to their planet, so long as they ceased to be in conflict with one another.

The Refusians once had physical form. A giant solar flare caused the Refusians to lose these bodies, and as a result, they no longer had a recognisable "being". The Refusians were not only invisible to other races, they were also invisible to one another; they could only sense another Refusian's presence. In their new forms, the Refusians were super-strong and could move heavy objects as if they were practically nothing. They were also an advanced race, possessing various weapons that seemed to have a wide range of capabilities, from stunning Monoids to blowing up landers, without even being seen to operate.

Despite their newfound talents, the Refusians missed seeing signs of life on their planet. Once they became aware of the Ark's journey to Refusis II, they welcomed it, and began constructing a city in which the humans and Monoids could live. They were dismayed to see the fighting between and among the groups when they arrived on Refusis, and agreed to help the Doctor defuse the situation. Once the humans had been saved from destruction, and the Monoids had quit fighting, the Refusians ordered them to begin living together in peace if they wanted to stay on the planet successfully. (TV: The Ark)

When the First Doctor suddenly turned temporarily invisible, Dodo Chaplet and Steven Taylor thought that it was caused by contact with Refusians. (TV: The Ark, The Celestial Toymaker)