The spirit pack pig is a Beast of Burden familiar that can be created in the machine under Eli's farm, after completion of the Bringing Home the Bacon quest. The familiar comes in 3 tiers. Each tier increases the usefulness of the pouch. To increase the tier of the pigs, one must upgrade the machine in the storm cellar. Each tier uses the same pouch. Upgrading the machine after obtaining a pouch still summons the higher-tiered familiar.
The tier 3 Pack pig has a storage capacity and duration greater than that of the War Tortoise, but a lower combat level. Unlike the tortoise, the Pack pig is non-aggressive. The cost of the pack pig pouch can't be calculated because the teeth are untradeable. However compared to the Tortoise, it requires 5 extra shards (125gp), and collecting pig teeth may take about as long as collecting tortoise shells. With the Tortoise however, you have the option of buying the shells or the pouches from other players.