My filly has a wavy tail and my last horse had a wavy tail. The weird thing is, there tails arent as...silky(i guess that's the word to use)as the horses with straight tails. My Paint filly has a wavy mane and tail and my bay QH just had a wavy tail. Its very interesting.
My appy has a straight tail, and so does my mare... His is like fine hair, you can usually just comb your fingers through it. My mare's tail on the other hand, despite it being straight, is so wirey and tangly that she gets dreadlocks quite often.
The Morgan's I'm working with right now ALL have wavey manes and tails. All of them. .__.;;
Jack has plenty of natural wave in his tail. It's pretty thick as well (wavy manes and tails tend to be). Oddly enough, it rarely gets bad tangles. A bit "stuck together" if you get what I mean, but nothing a little show sheen and a brush doesn't fix with minimal effort. His mane is very thick as well but is straight as a stick.