As per Ramayana Balakanda (Chapter 1 (16), all the vanaras or monkeys who fought against the army of Ravana along with Bhagavan Sri Rama were forms of gods in Hinduism. When Bhagavan Vishnu took the form of Rama on earth, Brahma directed the gods to take avatar or manifestations of vanaras and create powerful army for Sri Rama.
Thus on the direction of Brahma, various Devas appeared on earth and they were accomplished in may and bravery, with a speed that of is like that of the wind. They were highly knowledgeable and possessed supreme intelligence. Their bodies were divine and knew about all the weapons.
The vanaras were born from the bodies of apsaras, yaksha, pannaga (snakes), riksha (bears), vidyadhara, kinnara, vanara and gandharva women.
All the sons of the Devas were equal in valor to their father and mother.
To defeat Ravana and to help Sri Rama, Siddhas, rishis, serpents and celestial birds too gave birth to powerful beings.
The sons of these divine beings could assume any form at will. They possessed bodies that were like mountains and elephants.
They fought with rocks, trees, nails and teeth. They were all capable of disturbing the great oceans and shattering the ground with their feet. They could jump and touch the clouds in the sky. They soon numbered in millions. They chose Vali and Sugriva as their leaders.
They all are marched from different directions and finally reached the slopes of Mount Rikshavat waiting for the arrival of their Bhagavan – Sri Rama.