It is a lavender-colored jellyfish. It has no spots and is more translucent than the more common pink jellyfish. In "Tea at the Treedome," a lavender jellyfish is seen with four stingers. In other appearances, they seem to have six stingers.
This was the original jellyfish design that Stephen Hillenburg planned to use as the main breed of jellyfish in the series. It appeared in his series bible, as well as in early promotional artwork for the show. In "Jellyfishing," pink jellyfish were introduced, and they became the standard species used in episodes from then on.
SpongeBob finds one at the start of "Tea at the Treedome" while he is jellyfishing, but he fails at actually catching it. The lavender jellyfish from this scene reappears in the montage sequence of "The Sponge Who Could Fly" and in the book Gone Jellyfishin'.
A lavender jellyfish makes a cameo appearance in Jellyfish Fields in "Jellyfishing" and "F.U.N." One also appears when Sandy flies to the moon in "Sandy's Rocket."