After mucking around in arena mode for a while, I eventually ended up with a large creature that drops a shell when butchered; most of what I did to get this was copying the regular turtle, removing some vermin-related tags, and making it more like a large creature (the body size in particular wouldn't let it drop anything when I butchered it until I made it bigger). Whether you can use it like a normal turtle shell in fortress mode remains to be seen, but here's something for you.
[CREATURE:GALAPAGOS_TURTLE]
[DESCRIPTION:A giant turtle that lives in tropical shrublands.]
[NAME:galapagos turtle:galapagos turtles:galapagosian]
[CASTE_NAME:galapagos turtle:galapagos turtles:galapagosian]
[CREATURE_TILE:15][COLOR:6:0:0]
[PETVALUE:100]
[LARGE_ROAMING][BENIGN]
[AMPHIBIOUS][COMMON_DOMESTIC]
[NATURAL][PET_EXOTIC]
[BIOME:SHRUBLAND_TROPICAL]
[POPULATION_NUMBER:10:30]
[CLUSTER_NUMBER:3:5]
[FREQUENCY:10]
[PREFSTRING:mighty shells]
[BODY:QUADRUPED:TAIL:2EYES:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:SKULL:MOUTH:RIBCAGE:SHELL]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[REMOVE_MATERIAL:SKIN]
[REMOVE_MATERIAL:LEATHER]
[REMOVE_MATERIAL:HAIR]
[USE_MATERIAL_TEMPLATE:SCALE:SCALE_TEMPLATE]
[USE_MATERIAL_TEMPLATE:SHELL:SHELL_TEMPLATE]
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[REMOVE_TISSUE:SKIN]
[REMOVE_TISSUE:HAIR]
[USE_TISSUE_TEMPLATE:SCALE:SCALE_TEMPLATE]
[USE_TISSUE_TEMPLATE:SHELL:SHELL_TEMPLATE]
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SCALE:FAT:MUSCLE:BONE:CARTILAGE]
[SELECT_TISSUE_LAYER:HEART:BY_CATEGORY:HEART]
[PLUS_TISSUE_LAYER:SCALE:BY_CATEGORY:THROAT]
[TL_MAJOR_ARTERIES]
[BODY_DETAIL_PLAN:STANDARD_HEAD_POSITIONS]
[BODY_DETAIL_PLAN:SHELL_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_RIBCAGE_POSITIONS]
[USE_MATERIAL_TEMPLATE:SINEW:SINEW_TEMPLATE]
[TENDONS:LOCAL_CREATURE_MAT:SINEW:200]
[LIGAMENTS:LOCAL_CREATURE_MAT:SINEW:200]
[HAS_NERVES]
[SPEED:2900]
[USE_MATERIAL_TEMPLATE:BLOOD:BLOOD_TEMPLATE]
[BLOOD:LOCAL_CREATURE_MAT:BLOOD:LIQUID]
[CREATURE_CLASS:GENERAL_POISON]
[GETS_WOUND_INFECTIONS]
[GETS_INFECTIONS_FROM_ROT]
[USE_MATERIAL_TEMPLATE:PUS:PUS_TEMPLATE]
[PUS:LOCAL_CREATURE_MAT:PUS:LIQUID]
[BODY_SIZE:0:0:1000]
[BODY_SIZE:3:0:60000]
[CHILD:3]
[BODY_APPEARANCE_MODIFIER:LENGTH:90:95:98:100:102:105:110]
[BODY_APPEARANCE_MODIFIER:HEIGHT:90:95:98:100:102:105:110]
[BODY_APPEARANCE_MODIFIER:BROADNESS:90:95:98:100:102:105:110]
[MAXAGE:100:150]
[ATTACK:BITE:BODYPART:BY_CATEGORY:MOUTH]
[ATTACK_SKILL:BITE]
[ATTACK_VERB:bite:bites]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_CANLATCH]
[ATTACK_FLAG_EDGE]
[DIURNAL]
[SWIMS_INNATE][SWIM_SPEED:2500]
[MUNDANE]
[CASTE:FEMALE]
[FEMALE]
[CASTE:MALE]
[MALE]
[SELECT_CASTE:ALL]
[CHILDNAME:galapagos turtle pup:galapagos turtle pups]
[SET_TL_GROUP:BY_CATEGORY:ALL:SCALE]
[TL_COLOR_MODIFIER:PALE_BROWN:1:DARK_BROWN:1:FERN_GREEN:1]
[TLCM_NOUN:scales:PLURAL]
[SET_TL_GROUP:BY_CATEGORY:EYE:EYE]
[TL_COLOR_MODIFIER:BLACK:1]
[TLCM_NOUN:eyes:PLURAL]
Some tags and numbers you might want to change:
[CHILD:n] The age at which the turtle ceases being a child and reaches adulthood (i.e. sexual maturity).
[BODY_SIZE:n:m:xxxxxx] The third number is the size in cubic cm, the first two numbers are the year and days at which the creature reaches that size. Multiple bodysize tags cause the creature to grow in size over time; in this case the turtles start out the size of a newborn puppy and reach the size of a dwarf at age 3.
[BIOME:whatever] I put tropical shrubland, you can change or add additional ones using
this list.
[SPEED:n] Given some of the default speed values, some numbers from wikipedia, the speed equations on the wiki, and the assumption that a human can walk about 4mph, I extrapolated that the turtles would have a speed of roughly 20000. That number's rather insane, so I just kept it at the speed of normal turtles (2900). Creatures can move every ((rawspeed/100)+1) turns, so this speed is about three times as slow as dwarves and humans. There's also the swimspeed to consider.
Population number, cluster number, and frequency. I never really understood popnumber (wiki seems to alternate between "number of creatures that live on the map" and "number of creatures that show up per year"), so I made them slightly less populous than wild horses. Frequency is out of 100, so 10 is fairly rare.
Finally, there's no way I know of to make them withdraw into their shell (though the shell should at least absorb some blows that would otherwise hit a more sensitive area). Additionally, I haven't tested them to see if the shell can actually be used, but you should still be able to get it from butchering.