Glass Dalek

A Glass Dalek was a Dalek variant created by Davros for the reproduction of his Imperial Daleks. They were designed to turn humans into Daleks.

History
In a retelling of his first encounter with the Daleks and the First Doctor, Ian Chesterton claimed a Dalek with a glass casing acted as the leader of the Dalek City on Skaro. The Thals hacked it to pieces during an assault on the City. (PROSE: Doctor Who in an Exciting Adventure with the Daleks)

In Tranquil Repose on Necros, Davros used the Glass Daleks as part of his experimentation. By placing a cryogenically-frozen head into the casing, the head would mutate into the deadly creatures that made up the Imperial Daleks. When the head had become another Dalek, the Glass Dalek casing changed into that of an Imperial Dalek and another glass casing took a new head. Arthur Stengos almost became a Dalek before his daughter, Natasha, on his instructions, killed him. (TV: Revelation of the Daleks)

During the Last Great Time War, the Daleks gathered up humans and placed them in Glass Dalek casings which acted as incubators while the human inside was mutated into a Dalek, after which they would be placed into a standard Dalek casing. These Daleks were used as foot soldiers and cannon fodder as they were viewed as inferior and more expendable than pure-bred Daleks. The War Doctor and Cinder found five of these casings in a Dalek laboratory in one of their saucers at a temporary Dalek base in the city of Andor on Moldox. Cinder tore up the cables, allowing the people inside the casings to die in peace. (PROSE: Engines of War)

Behind the scenes
The unrelated Psyche Daleks had a transparent, enlarged dome, but an otherwise normal casing, (COMIC: Emperor of the Daleks!) and there are even earlier record of a Dalek leader in transparent casing (PROSE: Doctor Who in an Exciting Adventure with the Daleks)