Make it with a high speed and a high damblock tag. Make it small with the size tag to minimize the damage and don't put any attack tags in it. Put it in all biomes and have it occur frequently and in large groups. No sleep, pain, hunger, bleeding, nausea, breathing, stunning or thirst are always handy.
EDIT: Actually a good idea. I'll give it a try.
EDIT2: Here you go. I made the trainable so they can be used as a distraction when enormous enemies attack. That's also why I added some other tags to make the sturdier in case of GCS, dragons, HFS... Add this to any random creature file. You don't even have to make a new BODY-entry. I've only tested in Adventure mode, and it survived a cougar attack. I tried to retaliate and I could neither attack nor wrestle the thing. If there are any complications in Fortress mode, just let me know and I'll try to figure out what went wrong.
[CREATURE:DUMMY]
[NAME:training dummy:training dummies]
[TILE:'d'][COLOR:1:0:0]
[LARGE_ROAMING]
[trAINABLE][PET]
[PETVALUE:2]
[NO_DRINK][NO_EAT][NO_SLEEP][NOBREATHE][NOBLEED][NOEMOTION][NOEXERT][NOFEAR][NONAUSEA][NOPAIN][NOSTUCKINS][NOSTUN][NOTHOUGHT]
[BODY:BASIC_1PARTBODY]
[BIOME:ALL_MAIN]
[SPEED:5000]
[DAMBLOCK:1000]
[SIZE:3]
[FIREIMMUNE_SUPER]
[POPULATION_NUMBER:25:50]
[CLUSTER_NUMBER:5:10]
[CHILD:1]
[STANDARD_FLESH]
[LAYERING:250]
[HOMEOTHERM:10067]
[SWIMS_INNATE][SWIM_SPEED:12500]
[ALL_ACTIVE]
[AMPHIBIOUS]
[CANNOT_UNDEAD]
[BENIGN]
[GOOD]
[EVIL]
[LARGE_ROAMING]
[MEANDERER]
[NATURAL]
[PARALYZEIMMUNE]
[STOUT]
[WEBIMMUNE]