The problem with using an AirTag on an animal, other then the possibility that they could eat it, is that the AirTag is just like the other Bluetooth trackers that require another device nearby (usually about 30 feet) to be able to transmit the location. An animal that runs off where there is not another iPhone within that range will be unable to be located.
There is a difference between Bluetooth trackers and GPS trackers. The GPS tracker maintains a connection to a satellite when it has an unobstructed view of the sky and does not need another device to transmit the location. There are downfalls to GPS trackers too, such as:
- They are larger in size to account for the antenna
- They are more expensive
- They require more power, which usually means the battery will not last as long