There are a few ways you can make a new tail hole
1) If the pony has the rooted tail you can just cut where the current holes are and BOOM normal tail hole
2) If rooted tail, fill in with clay like apoxy sculpt, take a drill and drill in a new tail place where you want it
3) Like #2 only if no drill is available a screw driver and screw work just as well
If no apoxy sculpt is available, you might be able to get away with filling the holes several times with something like mod podge, triple thick, or possibly even air dry clay. You can also try to use a heated tool to melt the holes together but I have not try this