Pack yak

Pack yaks are level 96 Summoning familiars with the largest inventory among all the Beasts of burden. They have the capability of temporarily holding up to 30 items for a player, whose total value can be no more than 1,500,000 gp. Pack yaks also have a special move called Winter Storage, allowing instant banking. Similar to the Unicorn Stallion, a Pack Yak will only fight when attacked.

Pack yak pouch
A Pack yak pouch is made by using a Summoning pouch on a Summoning obelisk with 211 spirit shards, a Crimson charm and a yak-hide in the inventory. Making the pouch earns 422.4 experience points. Using the pouch to summon a Pack yak gains 4.8 experience points, and costs 10 Summoning points.

Winter storage scroll
Winter storage scroll enables the use of the Winter Storage special move for a Pack yak. Using the pouch on a Summoning obelisk creates 10 scrolls.

Winter Storage
Winter Storage is the special move for the pack yak, which when used on an item in the player's inventory, instantly banks it without the need of going to the bank in person. This allows players to camp monsters (like dragons) for hours, and bank items that the player would have otherwise left behind.

Trivia

 * In the original update, Pack Yaks could only hold ten items. This was changed to thirty, three times as much.
 * The Winter Storage Scroll can be used even in the high Wilderness to send items to your bank.