Mister Treasure is the yukkuri equivalent of a human toy, and as such denotes items owned by koyukkuris, and used in their childish games. In stories, "Mister Treasure" mostly appears in its slurred form of "Mishter Threashure", as a koyukkuri could say it.
Much like human child would antropomorphize and speak with its favourite toys, is not uncommon for a koyukkuri, especially a lonely one, speak with its "Mishter Treashure" as it were its best confidant. Also, koyukkuris may argue for the possession of a particularly prized "treasure".
Much as adult Yukkuri define their "easiness" by their beauty and material possessions, a koyukkuri may feel easier with a nicer "Mister Treasure". As such, in the wild a benevolent parent will be more than willing to allow a ko bringing its improvised toys in its den, and in captivity a good owner should always buy some toys for its pet koyukkuri.