I'm sorry to say I was slow to catch that, only realizing it about halfway through the season (around the Rooster episode), even with the intro giving a clear indication of what they were doing. The thirteen episodes represent the 12 traditional zodiac animals, and the 13th would have been the Year of the Cat that never officially became a zodiac animal - Fruits Basket fans tremble.
You could say my not realizing the theme until late, though, is a testament to how naturally the different animals were integrated into the plot. As such, I was impressed with the direction of the episodes. As always, how effective they are as creepy stories various for each person and also episode to episode, but I overall found it very enjoyable.
