I was just browsing the shop, looking at the spirit choices for custom binding vessels. I found a spirit I wasn't familiar with, called the Hounds of Annwn. Doing a quick search yielded the Cwn Annwn, which confuses me because Cwn Annwn is listed as a separate creature on the listing I saw. If these are two different spirits, can someone please tell me what the difference is?

Not sure of the difference tbh. But I must say I love my Cyn Annwn

I have both.. With mine, the hounds are a bit darker or moodier... but not by much. You really have to be paying attention to tell. (Others may have a different experience)
I love all of mine.

I too am curious about this. As far as I can tell Cwn Annwn is a species name but Hounds of Annwn refers to those specific hounds who serve or have served the Welsh death deity Annwn. A hunting dog breed is built to hunt but that does not mean ALL of them are used as hunters. I think that the Hounds of Annwn are listed as DA because of the job they did/do for Annwn. And actually Cwn Annwn from welsh translates to hounds of Annwn. Not all of the species serve/served Annwn as far as I can understand.