A group of foxes is called a skulk, leash or earth. The term "skulk" may be used for any animal considered vermin, but it is especially associated with foxes, which are indeed considered a vermin species in Europe and Great Britain due to their high numbers.
The word "skulk" primarily means to wait, lurk or move about stealthily. It comes from earlier Scandinavian words meaning to malinger, spare oneself or play truant. Foxes are notoriously evasive when being hunted, running into holes or up trees to avoid capture. The particular use of "skulk" for a group of them may come from this behavior. It should be noted that foxes rarely congregate in groups. Foxes are considered solitary animals, and although they may form family units when their offspring are very young, they generally do not travel in groups as wolves do.