Primitars were made out of primitives and used as avatars while Second Life was in beginning development stages (pre-alpha) before avatar meshes were introduced. Looking very much like futuristic astronauts, primitars were skeletally-animated humanoids with masks, visors, and jetpacks (with jetbursts when flying).
It is said that the primitar was the basis for the modern mesh-based system that was introduced in alpha. The central slider setting of 50 on the height slider today corresponds to the primitar's approximate height. The primitar is said to have been accidently made a bit taller than your average human, which is why placing the sliders in the middle make an avatar a bit taller than expected. This fact may also contribute to why the current avatars are taller than average.
This Wiki's icon (upper left corner) features one such primitar.
n00body Cain has recreated the primitar as an avatar that is freely available and fully modifiable. The primitar avatar can be found at Yadni's Junkyard and at the Historical Museum.