Saw one of these...interesting no doubt. Only once in my lifetime though...also saw a storm in a single cloud on a sky blue day. That was really entertaining to watch!
Parhelia (commonly called "sun dogs") come in pairs (usually to the left and right of the sun at a distance of 22 degrees) and form in high, thin clouds that are composed of tiny ice crystals.
That "standalone" storm you saw is also the most dangerous kind. Unlike T'storms in a front line, they can draw energy from all the surrounding air -- instead of having to "share" it with adjacent storms. Tornado factories!
Saw one of those "supercells" from the parking lot of Carlsbad Caverns, NM -- which is high up on a one-time seashore. The storm moved across the flat former seabed (now desert) below and to the south, and the falling rain and hail shafts were clearly visible. As it moved on, it left a dark, wet "footprint" behind it across the desert. The top must have been near 35,000 feet. Lots of lightning and noise -- quite a spectacle (when watched from a safe distance...)!!
As a "Skywarn"-trained Ham, I wanted to stay and see if it "dropped" a tornado -- but the rest of the family wanted to tour the caverns in time to be back on top for the evening bat flight. The kids won. ;-)