I assume that in this context the fox tail has an allegorical meaning. Which may be that of the Greek legends, where the fox's tail was a symbol of greed, cunning and treachery, as well as lust and whoring.
There is a lot of discussion of this on one of the 15th C areas. There seems to be evidence of one particular chap having one hanging from a lance as a standard. Someone started selling the things at fairs a few years back, a lot of the kids liked the idea of having a fox tail for fun but why the grown ups stared wearing the things is beyond me! A medern re-enactorism. There are lots of re-enactorisms dating from the early days -like Bill Drill. If you talk to the folks who were in early groups they freely admit adapting civil war stuff. To be honest, get a load of big blokes with long sharp things and you do need some sort of disciplined handling techniques so they probably used them for safety reasons. Having played with the Gloucesters and their sharps which they use for parade I am grateful for that. i still have all noses, ears and fingers I cam equipped with. Interesting to see a new re-enactorism like this develop though just as we thought we were starting to root out some of the worst excesses.