Tried to look at their tribe ID? If tamed, their tribe ID would be a 10 digits integer otherwise would be a short index (that I believe identifies the spawn region they spawned at or something).
Wild:
{
"DinoID": "9751250656956615",
"TribeID": 12,
"Species": "Pegomastax",
"Loc": [ -202299.390625, -43369.39453125, 75247.4609375 ],
"Level": 35,
"Base Stats":
{
"Health": 3,
"Stamina": 6,
"Oxygen": 5,
"Food": 7,
"Weight": 3,
"Melee": 5,
"Speed": 5,
"Crafting": 0
}
}
Wild (Thief):
{
"DinoID": "18058434149784246",
"TribeID": 12,
"Species": "Pegomastax",
"Loc": [ -204957.46875, -41341.8671875, 75247.2421875 ],
"Level": 35,
"Base Stats":
{
"Health": 4,
"Stamina": 3,
"Oxygen": 7,
"Food": 4,
"Weight": 5,
"Melee": 5,
"Speed": 6,
"Crafting": 0
}
}
Tamed (Thief):
{
"DinoID": "18058434149784246",
"TribeID": 1180221974,
"Species": "Pegomastax",
"Loc": [ -202238.015625, -43390.59375, 75247.5 ],
"Level": 52,
"Base Stats":
{
"Health": 4,
"Stamina": 5,
"Oxygen": 11,
"Food": 7,
"Weight": 7,
"Melee": 6,
"Speed": 11,
"Crafting": 0
}
}
I believe the game mark them as "tamed" as soon as they steal something so that they don't despawn with your loot if they run outside of their spawn.