Shaggydog
The wildest of the six, matched to the youngest and least-tended Stark.
The wolf itself
Shaggydog is black ÔÇö the only black one in the litter ÔÇö with a long unkempt coat that never lies flat and gives him the outline of something larger and less finished than his brothers. His eyes are green, wild and pale, and the combination of black coat and green eye in a dark stable is the reason grown men at Winterfell took to carrying a light. He grew fast and roughly. Where Summer moves silently and Grey Wind moves like a soldier, Shaggydog charges: he is all forward motion, shoulders first, and he does not stop when shouted at.
His temperament is rage, and it is not mysterious where it came from. He bit people. He bit a man of the household hard enough to draw serious blood, he could not be trusted in a hall or a crowd, and eventually he had to be kept in the crypts and the tunnels beneath the castle because there was nowhere above ground that could hold him. He snarls at approach, he snaps at hands, he takes food by tearing it off whoever is holding it. His voice is a high broken howling that goes on far too long and stops abruptly. He is not evil and he is not mad ÔÇö he is an animal that was never given the attention that made the other five manageable, belonging to a small boy who was in exactly the same position, and the two of them turned out precisely as anyone paying attention would have predicted.
The bond
Rickon was three when the pups were found and four for most of what followed, and he named his wolf the way a very small child names things: for what it looked like. That is the last uncomplicated moment in the arrangement.
The war took his father and his mother and his brothers and sisters out of Winterfell inside a year, and left the youngest Stark in a castle staffed by people who had a great deal else to worry about. Nobody raised him. He was told very little, understood more than anyone credited, and became a difficult, biting, screaming child who ran wild in the crypts ÔÇö and his wolf, the only creature in the castle that was entirely his, did precisely the same. The warging between them is the least documented in the family and probably the most complete, because there was nobody there to teach either of them where the line was. Rickon dreamed about his father before anybody told him anything, and said so, out loud, to adults who did not want to hear it.
The wolf is a fairly exact portrait of what was happening to the boy: left behind, half-forgotten, and getting feral by the month.
Loose in the North
When Winterfell stopped being a safe place, the two youngest Stark boys were taken out of it by a handful of loyal people and split up deliberately, on the reasoning that two brothers travelling together are a single prize. Rickon and Shaggydog went one way with a wildling woman who had every skill required to keep a wild child and a wild animal alive in open country, and who was, notably, one of the very few people the wolf would tolerate.
Where they went afterwards the story keeps deliberately vague for a long stretch ÔÇö north and east, into country where the writ of the southern war does not run at all. It is one of the few threads in the whole saga that simply drops out of sight for years, which is unusual enough to be worth noticing.
What can be said is that a black direwolf running loose in the north with a small boy attached to it is a considerably more dangerous object than either would be alone, and that the people who eventually took an interest in the last free Stark heir did not do so because he was harmless.
What became of it ÔÇö if an ending is written, it is here. Spoilers.
Shaggydog was killed and his head delivered to Ramsay Bolton as proof of Rickon's capture ÔÇö a wolf reduced to a receipt.
It was done by a northern house that had spent the war being careful, calculating exactly which way to jump, and decided that a boy and a dead animal were the price of the jump. The pattern holds for the last time: separate the Stark from the wolf, and what follows becomes possible.