You may want to check and see if part of the tailbone is missing. That tail closely resembles Mudpie's, and he is missing half of his tailbone, probably due someone leaving a braid in for a long period of time, which resulted in the hair getting wrapped around the tail bone and then cutting of blood supply. That's mine and my vet's best guess, anyways. When I first discovered it, I was a little concerned, but it causes him no pain, it just makes it so his tail will never be as full as most horses's can be.
I went on a mad mission to make it longer, and it's been going pretty well! It's longer now than it's ever been before. What I do is I wash the tail, condition it, and then keep it in a tail bag. I'll repeat this every weekend. Keeping it up all the time definitely helps!
Here it is about a month after I started this:
Here it is last weekend... I actually didn't have a tail bag in at all for about two months, but it didn't do too much. This is right after I washed, conditioned, and then put detangler in it:
This is his tail in one of his tail bags:
He's also on SmartOmega 3, which has made his hair healthier, shinier, and faster-growing. :)
EDIT – You can't really see it too well. Here's another picture of it. It was dirty and unbrushed, but you can kinda see the length a little better: