Hanuman was on the flag of Arjuna. Hanuman represents power and strength.

Arjuna Vishaada Yoga

Chapter 1, Verse 20

atha vyavasthitan dṛṣṭvā 

dhārtarāsṭrān kapi-dhvajaḥ 

pravṛtte śastra-sampāte 

dhanur udyamya pāṇḍavaḥ 

hṛṣīkeśam tadā vākyam 

idam āha mahi-pate

Then, beholding the sons of Dhritarashtra standing in battle order, and the flight of missiles having begun, the son of Pandu (Arjuna), whose emblem was an ape, took up his bow and spoke these words to Sri Krishna, O King:

Hanuman was on the flag of Arjuna. Hanuman represents power and strength. Sanjaya showed this to Dhritarashtra saying, "As promised, Hanuman has placed Himself on the flag of Arjuna, and from time to time, Hanuman roars from the flag." Sanjaya used the word kapi-dhvajaḥ meaning "from time to time he roared", showing that Arjuna had the attribute of strength.

But seeing that all the Kauravas were ready with their weapons to begin the battle, Arjuna started to feel sad. Certain emotions started to take over inside his heart. He spoke to Krishna again calling Him, hṛṣīkeśam, 'the controller of the senses Using the word hṛṣīkeśam, Sanjaya was saying that the controller of the senses, the knower of all hearts, was Him, the charioteer of Arjuna. He was reminding Dhritarashtra that the Pandavas would win. When a good quality awakens, the Divine is present. And if this quality is energised with bhakti, with devotion, then the Lord Himself will be one's charioteer. Being the charioteer means that the Lord is the power, the strength. When one has the willingness, it is the Lord who gives the power to move forward. So, Sanjaya reminded Dhritarashtra that Arjuna had God as his charioteer and had Hanuman on his flag. Wherever there is Hanuman, nothing can go wrong. He is the protector, the strength, the ocean of wisdom. And with that fully, one can expect victory! Hanuman was powerful and completely surrendered, as was Arjuna. Hanuman was surrendered to Lord Rama; Arjuna was surrendered to Lord Krishna. Here Sanjaya was reminding Dhritarashtra that this charioteer was not a normal person: it was God Himself sitting there in the form of Sri Krishna. How could there be defeat? There could not be any defeat! There would be only victory!

Bhagavad Gita