I don't know what you are supposed to do with these, but they sure sell for a lot to NPC vendors. I had 13 or so of these stacked after hunting centaurs, sold for 304pp all together.
Somewhat less relevant, but Centaurs generally drop equipment like weapons, shield, bow and while they make good pet weapons (finesteel equivalent) they are all crap money wise (sells for like 7sp to vendors). So I would recommend not bothering with those and instead keep these tails and also hoofs (1pp). Much lighter and less cumbersome in the inventory. Trade skill items drop as well, but you would have to sell them to players as NPC merchants won't buy most of those, with the possible exception of Simple Binding Powder (2pp).