I assume by fatbeard he's refering to a "bloatie"-- a beard grown with the intention that the lower edge will give some definition to a lower jaw that that would otherwise have little visible distinction from the overweight (or just unusually thick-necked) individual's neck.
A neckbeard is often seen on some teenagers and others trying in vain to grow a decent beard. The cheeks and chin on such people may have such sparse, straggly hair that many would argue they have no business attempting a beard, but they nevertheless try to compensate by letting their denser neck hairs grow out too long, with the result that they end up bearing a strong resemblance to
this fellow. Also known as a "throatie".