According to White Bull the Santee who took Custer's horse just after the battle at the Little Bighorn was Noisy Walking (aka Noisy Walker aka Sound the Ground as He Walks). He was the son of the Wahpekute chief Inkpaduta, the leader of the Santee and Yanktonais camp at the LBH. Maybe Grey Tracks was another name of him.

His twin brother Tracking White Earth was wounded at the battle and later died of his wounds in Canada.
Charles Eastman, who was related to Inkpaduta, said that “for twenty years the Western Sioux claimed that one of his sons killed Custer. It is possible, but there is no proof. (…) it is true that the warriors of the western Sioux had the greatest regard for the bravery of Inkpaduta´s sons in battle.”

Inkpaduta and his band were much hated by the Americans, no wonder his descendants hid their identity and knowledge about the Custer fight in fear of retaliation.

After the Little Bighorn Battle Inkpaduta (or Red Top) and his band fled to Canada with Sitting bull.
There Inkpaduta died in 1881. — Karl

There were several descendants living in Canada, even in 1934 there were two of his sons, Little Spirit (Wanagi Ciquana) and Charley Maku and also a daughter living near Pipestone Reservation in Manitoba.

The best sources I have about Inkpaduta and his sons are Doane Robinson´s “History of the Dakota or Sioux Indians” (Ross & Haines, Minneapolis 1967) although he presented Inkpaduta like a kind of villain, and Maxwell Van Nuys´ “Inkpaduta – The Scarlet Point” (1998) who gave a more balanced view of the Santee chief. — Dietmar Schulte-Möhring

Inkpaduta had many children, including two sets of twin sons. One set of twins Gray Earth Track (AKA Sounds the Ground When He Walks, and Noisy Walking) and White Earth Tracking chased the trooper who fled from LBH. White Earth Tracking was mortally wounded by the trooper. Gray Earth Track shot the trooper to death and took his horse. He later claimed the man he had killed (some miles from the battlefield) was Custer and the powerful sorrel horse he captured was Vic. Of course the Indians did not know it was Custer until later, so the claim came much later. This son of Inkpaduta kept and rode the white faced, white stockinged horse for another twenty years. Of course, it was not Vic. — Walt Cross

I heard that Gray Tracks/Sounds the ground when he walks/Noisy Walking died in Canada in 1878 or around that date, his brother kept the horse. The descendants were enrolled at Standing Rock and Cheyenne River.

Inkpaduta's daughter married a Bull Ghost and End Of Horn here on Standing Rock. Two sons were enrolled at Cheyenne River, one son enrolled at Spirit Lake and one enrolled at Sisseton.

One of Inkpaduta 's wife and two children was with the group of Ihuntonwan at Whitestone, when the massacre happened. The son was called Little Ghost.

Inkpaduta is considered a hero to many native people, We honored him by keeping his family safe. Today we can talk about his relatives and who they are but even 50 years ago no one would speak his name in fear of the government. Today we have young people named after him to celebrate their victory in war.
— Ladonna Brave Bull Allard

Here is the photo and some text from the book of Mark Diedrich:

INKPADUTA, date and photographer unknown. This photo, published here for the first time, was only recently discovered, and is purportedly of Inkpaduta, the notorious Wahpekute Dakota chief. No known photograph of him was thought to exist. Yet, this slightly out of focus portrait portrays an Indian who fits Inkpaduta´s general facial description -- long, slim face, with high cheekbones, sunken sully eyes, and a large mouth with unusually big canine teeth. If it indeed is Inkpaduta, it probably was taken before the 1857 Spirit Lake massacre, and it was apparently turned up by Frank Heriott, an authority on the Inkpaduta troubles in Iowa. (from Mark Diedrich: "Famous Chiefs of the Eastern Sioux" Coyote Books 1987)

Well, if it is Inkpaduta at all, I don't think the photograph could have been made before 1857. I haven't seen many photos taken "in the field", when photo techniques hadn't developed so far. — Dietmar Schulte-Möhring