The SKARDIS was a time ship built by the Skith based on the Doctor's TARDIS. They were able to build it from the Doctor's knowledge of TARDISes when they probed his mind in 1915. The SKARDIS interior resembled that of the Tenth Doctor's TARDIS, but with an icy appearance and complexion. According to the Skith General, he didn't decide what "SKARDIS" would stand for, and just liked the name.
Unlike TARDISes, the SKARDIS was more primitive, and whenever it was activated, instead of travelling through time it would release a chronal wave. The SKARDIS was destroyed by UNIT. (COMIC: The Age of Ice)