I have seen this on the underside of the hide when skinning caribou, encapsulated in connective tissue. They are also called warble flies.
Interestingly, these were traditionally eaten by native people right at the time of butchering. Google it and you will see...
When they eventually emerge from the skin on live animals, the hides are full of holes until healing occurs. This was taken into consideration for deciding when to hunt if you wanted the hides for clothing, avoiding the time around the hatch. Vilhjalmur Stefansson wrote about this in his books on living with the Alaskan and Canadian natives for extended periods between 1906 and 1918.