A turtle is a type of reptile with a hard outer shell as its primary feature. The Chelon race resembled large saber-toothed turtles.
The inhabitants of Floe I resemble Earth turtles while on the surface of the planet. They loose the protective shell when they enter warmer climates. (TOS comic: "Ice Journey")
In Japanese mythology, the turtle represents longevity and good luck. (TNG short story: "Bedside Matters")
Cervantes Quinn noted that the Dulcinea handled like a turtle slogging through the mud while traveling through the wormhole to the Jinoteur system. (VAN - Declassified novella: The Stars Look Down)
Starfleet officers jokingly called the K'el Ri'anda class starship the turtle due to the ship's relatively poor performance and bloated appearance. (FASA RPG module: Klingon Ship Recognition Manual)
On Ceti Alpha V before the planet had been laid waste, Marla McGivers and Zuleika Walker encountered a giant turtle. (TOS novel: To Reign in Hell: The Exile of Khan Noonien Singh)
On the planet Droplet, Melora Pazlar had wondered if a series of floating islands were some kind of giant turtles. (TTN novel: Over a Torrent Sea)