Pack yak

Pack yaks are level 96 Summoning familiars with the largest inventory capacity among all the Beasts of Burden. They have the capability of holding up to 30 items for a player, whose value can be no more than 32,000 gp each (960,000 gp in total). Pack yaks also have a special move called Winter Storage, which allows for instant banking, making it a popular familiar to take to most areas. Similar to the unicorn stallion and bunyip, a pack yak will fight only when attacked.

Pack yak pouches are cheaper than their lower level counterpart, the war tortoise pouch, and also more effective in dealing with bosses that attack familiars if attacked by them. It is best to use them in places where a bank is very far away to reach.

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 for visiting a bank in person. This allows players to remotely bank items while remaining in a certain area for extended periods of time. When using the scroll on noted items only one item will be banked per scroll. The special move is also used while player killing because the special move can bank the gear of the player, thus not risking the items should they die. As the familiar lasts for 58 minutes, summoning the pack yak with a full special moves bar provides 1800 special move points over the course of its lifespan, allowing you to potentially use the Winter storage scroll 150 times. In cases where one will acquire more than 150 drops within that one hour, alternative methods such as Magic notepaper may be necessary for banking in addition to the Pack yak.

Casting the spell Make Leather and crafting dragonhide bodies will allow the yak's inventory to hold 90 dragonhides' worth of dragonhide bodies, and so players wishing to train at Green or blue dragons may wish to use this method to bank all of the dragon bones and dragonhides that the dragons will drop. To maximise efficiency, players should use Winter storage to bank the bones immediately, cast Make Leather 3 times when 15 dragonhides are acquired, craft 5 dragonhide bodies and store them in the yak, then repeat the process.

Winter Storage may be used in high-level Wilderness to send items to the player's bank, despite similar limitations to teleportation.

And no, you cannot cast Winter Storage on a Winter storage scroll.

Trivia

 * In the original update, pack yaks could only hold 10 items, however the items would stack.
 * Occasionally, you might see the pack yak say "Baroo baroo!" which is odd because the regular yaks on Neitiznot say "Moo".
 * After an update, you can no longer use winter storage scrolls to bank items in the Crucible, giving a message saying, "You cannot use that spell whilst in the Crucible."
 * There is currently a glitch with the Yak's scroll move on the Action bar, where once clicked it says "Null Familiar -> Item" rather than "Cast Winter Storage -> Item"

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