They definately kos. In fact, I had just logged into the game and a yk spawned just outside the inn I had camped in. So instead of me being safe, the ******* runs through the wall and proceeds to hack me numerous times for roughly 10-20 dmg. He ended up killing me *just* as i sought refuge from Seargant Slate who promptly killed the beast...3 seconds too late. Watch for these guys near Inn 3.
These things are very scary and deadly if you are level 8 and under! Depending on your AC and Agility, but mostly these things hit a max of 22. They also go by the name YK's.
I once conned them with my Erudite Wizard and it was Threateningly and i died a couple times because of this. Then when I play my Wood Elf Ranger they were Apprehensive to me. So always con them before you hunt around them because they may or may not attack you on site.
If you are a ranger or druid they will not attack you, same for most animals. Just be sure not to attack too many of them or they will start conning threateningly.
When these things spawn, at least in West Commonlands, they hang in the air for a little while. If you see one, avoid it because it is not just one, it is anywhere from two to four young kodiaks.
Will o wisps skelly's and Kodiaks do the float thing too in wc and ec I walked up to a willowisp and hit it and suddenly Boom I was half hp in what should have been one hit (turns out 7 of them spawned on me)