Magnedon

Magnedons, or Magnodons, were metallic, lizard-like creatures native to Skaro.

Biology
They were similar in shape to a crocodile, but with eyes on stalks and spines along their back. Their bodies were held together by an internal magnetic field and the First Doctor believed that they could attract other metallic creatures to prey on them. (TV: The Daleks)

Their flesh was made of living metal. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

History
Many magnedons were killed during the Thousand Year War, though their bodies were kept in relatively the same conditions. Their metallic physiology prevented them from decaying, thus leaving them intact. The Thals used their bodies to recharge their hand lights. (TV: The Daleks)

The Seventh Doctor encountered a weaponised Magnedon. (GAME: Dalek Attack)

A magnedon was kept on Station 7 by the Space Security Service in the petrified jungle, where it was found by the Eleventh Doctor and Amy Pond. (COMIC: The Only Good Dalek)

Behind the scenes
A strikingly different magnedon (though not named as such) also appeared in the Dr. Who film Dr. Who and the Daleks, which was a remake of The Daleks. The magnedon appeared as a huge petrified lizard-like beast. The exemplar featured was destroyed when Ian Chesterton fell into the pit where it was.

Like with other creatures and concepts from early Dalek lore, the spelling of the word "magnedon" was long in a state of flux, with the early Dalek annuals using the spelling "magnodon". As late as 2020, "magnadon" was used in behind-the-scenes material for the webcast A return to Skaro for the First Doctor... as Chris Thompson reflected on being "unsatisfied" with the way his magnedon animation had turned out.