When you lead a horse into a nether portal, when you come out on the other side there is 2 leads.

How to re-create
1: Make a nether portal
2: Tame a horse and put a lead on it
3: Walk through the portal and make the horse walk into it
4: Go to the nether
There will now be a horse and 2 leads on the floor
(Also sometimes the second lead appears in the over world)

This bug was discovered by me on a server, not sure if affecting single player