The Gruffs are a fae kindred, usually found in the service of summer and are hard core combat specialists, normally focusing on the killing of trolls but usually happy to kill whatever they are assigned to hunt.
Gruffs take the physical form of a humanoid goat, albeit with the horns curved forwards rather than back. Unsurprisingly for goats, they also smell very strong. They come in a range of sizes from around five feet tall to over twenty, and generally they keep coming until their prey is dead. Defeating one tier of gruffs just cues you up as a target for the next. According to custom they tend to attack in escalating order of power - first you get a pack of little gruffs, then a small group of larger ones. These will generally be followed by one enormous gruff (dubbed "Tiny" by Harry) and then, if the target is still alive, they are set upon by the Eldest Gruff - human-sized but an extremely powerful wizard1. Eldest gruff's power is underlined by the three purple stoles hanging from his belt - each the symbol of office of a member of the Senior Council that he has defeated. Very few people get as far as needing the attentions of Eldest Gruff.
Although all gruffs appear to be sapient, actual levels of intelligence may vary - the juniors appear cunning, but not particularly smart whilst regular gruffs seem at least at human levels and the eldest brother is presumably very smart indeed. Likewise, equipment varies from wooden clubs, to submachine guns, crystal armour and a sword as long as a car. All gruffs also appear to be extremely strong, fast and dextrous but, being true fae, suffer from the iron-bane. So far all the gruffs that have appeared in the series have been male and have considered one another brothers - females may or may not exist in this kindred.
Gruffs have only appeared to date in Small Favour.