Because the Picos are so close to the ocean, humidity is relatively high, and the peaks suddenly jut out from sea level, when the clouds move in, they create fog - very dense fog. If you can, try to get a weather report before you start out on a hike. We did from our inn owner who told us morning would be great - in fact there wasn't a cloud in the sky at first, but the weather was predicted to turn in the afternoon. He was right. Around lunch time, heavy clouds moved in and we knew we shouldn't linger too much at the upper elevation.
The Picos are used heavily for grazing so odds are you'll come across many cows and some horses. The Asturian livestock aren't scary, per se, but do watch where you step. If you don't, you're liable to land in a cow pie at some point.