Further to the post from Ian about deer, there are Sika deer at the Teifi marsh, and probably further around the area. They are similar (to my eye) to red deer, so beware which you see.
I have seen roe deer over the border in Pembrokeshire. I gather red deer are about too, which Ian confirms.
There is nothing quite like the shriek of a Sika at about 4o'clock in the morning when you are not expecting it. It is spine chilling!