It is any BoB pouch like the pig or the thorny snail?
You can use any Beast of Burden (BoB) pouch with the Warnborn Behemoth.
As quoted from the wiki
Use a Beast of Burden familiar pouch on your pet to grant it the according capacity. You must have the necessary Summoning level to use the pouch, and you can use more pouches to stack or renew the duration up to 2 hours.