An organism that is closely related to a mouse than to a turtle would be any member of the class Mammalia (mammals). Mice belong to the order Rodentia within the class Mammalia, while turtles belong to the class Reptilia.
For example, a rat, which is also a member of the order Rodentia, would be more closely related to a mouse than to a turtle. Other examples include other mammals such as rabbits, bats, or primates. All of these organisms share more recent common ancestors with mice than with turtles, aligning them more closely in terms of evolutionary lineage.