Pack yak

Pack yaks are level 96 Summoning familiars with the largest inventory among all the Beasts of burden. They have the capability of 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 and Bunyip, a Pack yak will fight only 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. Each pouch summons a Pack Yak which stays alive for 58 minutes before needing to be re-summoned.

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. The special move is also used for PKing because the special move can bank the user of the Pack yak's gear when they are about to die.

Trivia

 * In the original update, Pack yaks could only hold 10 items.
 * The Winter Storage Scroll can be used even in the high Wilderness to send items to your bank.
 * Occasionally, you might see the Pack Yak say "Baroo baroo!" which is odd because the regular yaks at Neitiznot say "Moo".
 * The Pack Yak is one of two beasts of burden that don't use gold charms, the other being a Spirit Kalphite, which uses Blue charms.