You can get a tail bag or give it a good brush and braid it. I have a pony with a thick/very dense tail and it's long and right now it's pure mud in the pasture and it did decently.

Tailbags help. or braiding but not to tight. As for thin tale I use tea tree oil on my mare she's got a horrid thin tale and i has thickened it so much. Not every day use though. I apply about 4 times a week