The Skaro Degradations were Dalek forces with abnormal and malformed appearances created during the Last Great Time War, which posed a greater threat than the original, unmodified species. The Degradations were created by the Daleks during experiments on Daleks from alternate timelines, inspired by the Fourth Doctor's attempt to prevent the Daleks from ever existing. The results created abominations of the Dalek form. The Daleks put the Degradations to use against the Time Lords during the Time War. Many were unstable and unpredictable. (PROSE: Engines of War)
They were described by the Tenth Doctor as one of the horrors of the Time War, along with the Horde of Travesties, and the Army of Meanwhiles and Neverweres led by the Could've Been King. He also cited an even more heavily altered Dalek, the Nightmare Child. (TV: The End of Time)
"Gliders" was the name used by Cinder and her fellow resistance fighters on Moldox to identify a type of Degradation that travelled by hovering above the ground. The Dalek mutant, resembling a limbless humanoid torso, was housed in a reinforced glass casing suspended under a regular Dalek dome. They had arms and energy weapons flanking them on either side. (PROSE: Engines of War)
Another type of Skaro Degradation, which somewhat resembled the Spider Daleks of the N-Space Dalek Empire, were known as "Spiders". They were egg-shaped and moved on three spider-like legs. Their casings were red with black sense globes and they were armed with four energy weapons. (PROSE: Engines of War)
The Time Lords' attempt to interfere with the creation of the Daleks gave the Daleks the idea to conduct further experiments on their own race. They started entering as many alternative realities as they could and began tampering with both their history and their DNA to create a powerful weapon they could use against the Time Lords.
This ultimately resulted in the birth of the Skaro Degradations which were used by the Dalek Empire by the time of the Last Great Time War. (PROSE: Engines of War) The degradations were first deployed into the War a short time before the Battle of the Althos system. Time Lord General Artarix was mortified by the degradations after encountering them. (PROSE: Decoy) They were used during the Dalek occupation of Moldox where the War Doctor encountered them. (PROSE: Engines of War)
When the Saxon Master broke the time-lock on the War as part of Rassilon's plan for the Ultimate Sanction, the Skaro Degradations were among the horrors of the War that threatened to break through. The Doctor and the Master were able to thwart Rassilon and re-seal the time-lock. (TV: The End of Time)
Earlier in the Time War, General Kallix reported to Cardinal Ollistra that his Time Lord forces had fought "the Spider forms of the Fifth Skaro Devastation" amongst numerous other Dalek variants, during their battles on the planet that the Neverwhen was detonated above, as a result of the time flux caused by the Neverwhen's detonation. However, Kallix did not state if these Daleks were of the Skaro Degradations. (AUDIO: The Neverwhen)