Oh dang it, I forgot a subentry I wanted to add!
The original White Dwarf Fiend Factory version of the greater urchin lived in dungeons and could fly with - it had Movement: 12″ (18″ flying).
That's too tempting not to include:
Flying Greater Urchin
These rare relatives of the greater urchin fly in a similar manner to a beholder and are equally at home in air and water. They live in dark, damp and spacious areas such as "wet" cave systems, dungeons and subterranean oceans. Flying urchins are much more dangerous than their marine cousins. Not only will they actively hunt larger prey, they are intelligent enough to fly out of melee reach while shooting spines at their opponents.
A flying urchins has identical statistics to a greater urchin except for the following changes:
Speed: 30 ft., fly 50 ft. (good), swim 50 ft.
Special Qualities: Amphibious, all-around vision, blind, clairvoyance, blindsight 60 ft., flight
Challenge Rating: As greater urchin +1
Environment: Underground
Flight (Ex): A flying greater urchin’s body is naturally buoyant. This buoyancy allows it to fly at a speed of 50 feet. This buoyancy also grants it a permanent feather fall effect (as the spell) with personal range.