A statue was a sculpture of a life-form, normally of its entire body.
A Melkur was an evil creature immobilised by the tranquil nature of Traken. Over time they turned into statues and disintegrated. The Master's TARDIS disguised itself as a Melkur as part of his plan to take over the Source. (TV: The Keeper of Traken)
A giant statue of a human was carved out of gregarian rock on the Ark during its 700-year journey to Refusis II, but was given a Monoid head when they took over. One had a bomb planted in the statue's head to destroy the Ark and the human passengers, but it was thrown out of the launching bay by a Refusian. (TV: The Ark)