What BoB pouch do I use with the war born behemoth?

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.

• However you must have the required Summoning level to use the summoning pouch, so the highest BoB pouch you can use on the Warborn Behemoth is the also the highest BoB familiar you can actually summon.

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.