Seasonal moult affects most of our native mammals, but is naturally most obviously evident in those with normally bushy tails ... foxes and squirrels.
Hair lost in the early part of the year is normally regained by the end of autumn, in good time for the onset of winter.
Hair loss on the tail and on the flanks may however also be indicative of an adrenal gland problem, caused by the overproduction of the hormone cortisol ... this is commonly called "Cushing's syndrome" and is normally a disease accompanying old age.
In dogs/foxes, this may be further evidenced by an animal that appears overly thin, but has a pot-belly. Sometimes the condition can have associated neurological symptoms ... look out for any tendancy for the fox to press the top of its head against a hard surface (post/paving slab/fence, etc).