K-9 was a chirpy little robotic dog (voiced by John Leeson) that joined the cast of Doctor Who in 1977. He's been through three iterations and it (he?) has returned in 2006 to join the tenth Doctor on his travels. Let's hope he doesn't try to hump
Rose's Martha's whoever's leg.
"The initial idea for realising K-9 was to use a small actor inside a robotic Dobermann costume, but that was rejected in favour of a radio-controlled prop, designed by Tony Harding and made by the BBC Visual Effects Department. The robot suffered from numerous technical problems during its time in the series, often malfunctioning because the radio controls interfered with the cameras and vice versa. On location, K-9 also proved unable to traverse uneven terrain, and shots had to be conceived with this in mind. Workarounds included using a concealed piece of twine to pull the character along, or laying wooden planks on which it could roll." (from Wikipedia)
K-9 Makes a Comeback (Gizmodo)