One of the followers of Barahir who remained with his lord after Morgoth overran Dorthonion. As Sauron's hunters pursued the outlaws, Arthad was one of the last twelve companions to remain with their lord at Tarn Aeluin. When Gorlim betrayed them, the hunters descended on Barahir's camp by the mountain lake, and slew all they found there including Arthad.
The arth- element of Arthad's name is probably 'realm' or 'kingdom' (as in Arthedain, interpreted 'realm of the Edain'). The final -ad might have various meanings; for example it might indicate 'place' or 'two'. In this context, however, is probably a simple name-forming suffix, so Arthad's name would mean something like '(Man of the) realm'.