A K-9 is a type of dog-like robot from Doctor Who. Originally invented by a scientist who could not take his dog with him to Titan, the Doctor travelled for a time with the original and another model, and gave two more to Sarah Jane Smith.
Toy version of K-9 Mark II. Available on ThinkGeek!
Mark I[]
The very first K-9 was made by Professor Marius because weight restrictions prevented him from bringing his own dog with him to Titan. These same weight restrictions prevented him from taking K-9 to Earth, so K-9 travelled with the Doctor and Leela, leaving the TARDIS with the latter after the Sontarans were kicked off Gallifrey. This model's ears are always swiveling.
Mark II[]
The K-9 Mark II only swivels its ears when processing data or a command. The possibility of laryngitis is built into its programming, resulting in a changed voice when the ailment passes. This is the first model to come with a dog whistle.
Mark III[]
This model has front and back wheel drive, a metallic blue paint job, and a handle to facilitate carrying. It can also function as a durable Canon Analysis Device.
Sarah Jane Smith's K-9[]
Sarah Jane Smith received a K-9 in a crate from the Doctor, many years after dropping her off in Aberdeen. This K-9 eventually broke down, but was repaired in time to sacrifice itself to stop an alien menace.
Tawaki and Tadkeeta's K-9[]
Agents Tawaki and Tadkeeta Penguin received a Mark III as a wedding present, from Omicron in Kamkenta's name. This K-9 developed a glitch that kept it from using first person for a time. It has also shown a certain aversion to cat disguise.
Mark IV[]
This model has a hover mode, red glowing eyes, and a number of other improvements.