While growing up B'Elanna Torres was made fun of for being half Klingon. She was once called "Miss Turtle Head" by Daniel Byrd, an insult which referred to her cranial ridges. Tuvok later used the same insult against her during a meditation session in 2375 as a means of demonstrating that she was easily provoked. (VOY: "Juggernaut")
Types of turtle
During the production of Star Trek: The Next Generation, the nickname for Michael Dorn's Klingon prosthetic forehead was "the turtle". Make-up artist Michael Westmore deliberately based his alien designs on things that can be found on Earth so they're recognizable to the audience; "If people are familiar with certain designs, maybe the shape and coloration of a butterfly wing or the shell of a turtle, it has a subconscious resonance that something completely unrecognizable won't have." (The Magic of Tribbles: The Making of Trials and Tribble-ations; Star Trek: Aliens & Artifacts, p.60)
In the Star Trek: The Original Series comic Ice Journey the inhabitants of Floe I resemble Earth turtles while on the surface of the planet, however they loose the protective shell when they enter warmer climates.
In the Star Trek: Vanguard Declassified novella The Stars Look Down, Cervantes Quinn notes that the Dulcinea handles like "a turtle slogging through the mud" while traveling through the wormhole to the Jinoteur system.
The Klingon Ship Recognition Manual, from FASA, states that Starfleet officers jokingly called the K'el Ri'anda-class starship "the turtle" due to the ship's relatively poor performance and bloated appearance.