I was really distressed when I looked at the achievements list and saw there was one for petting all the yaks in the village, because I had not realised it was possible to pet the yaks and by then the village was under attack. Thank goodness for the chapter select function. If you go up to them and press Triangle, Nathan pats them on the side and asks them how they're doing. Also, I didn't realise this until just now (I was trying to find a video of the yak-petting), but you can also make Nathan stand near the yak's backside and regret it.