We had an extra one that we put on our dog just to see how it went. I walked him for one mile using my own fitbit as the standard, then did the math and modified his stride length and walked him again. It took a few "calibrations" but we finally got one that was pretty close.
Stride length was all I changed. I wasn't worried about being exact but it turned out everything else was consistent enough that we could get a fairly trustworthy reading within a few tenths of a mile.
I don't know how it would work with an animal that didn't have a regular gait, like a rabbit or cat. Animals like dogs, horses, cows, etc would probably be more consistent. Certainly it would be a way to measure active/inactive times.
The wife made me take it off because it was getting his fur all matted around the collar and I didn't want to rig up something more permanent.