The eagle
Jacob is the eagle bearer or 'aquilifer' of the Twelfth Legion. It's supposed to be a great honor, but given that it involves carrying an empty pole - the Eagle having been lost in Alaska - Jacob is less than thrilled by his assignment, to him it is just an embarassing reminder that their cohort is shamed.
It is unknown whether he is still the eagle bearer after Percy and his friends restored the eagle after the quest.