There was a reference to Kesari, among all the vanara generals, who came in support of Sri Rama to Lanka, in Yuddha Kanda of Valmiki Ramayana.
Sharana, Ravana's aide, describes the individual strengths of the various commanders in the army of monkeys, mentioning their names one by one, in detail.
भ्रमर आचरिता यत्र सर्व काम फल द्रुमाः || ६-२७-३४
यम् सूर्य तुल्य वर्ण आभम् अनुपर्येति पर्वतम् |
यस्य भासा सदा भान्ति तद् वर्णा मृग पक्षिणः || ६-२७-३५
यस्य प्रस्थम् महात्मानो न त्यजन्ति महर्षयः |
सर्वकामफला वृक्षाः सर्वे फलसमन्विताः || ६-२७-३६
मधूनि च महार्हाणि यस्मिन् पर्वतसत्तमे |
तत्र एष रमते राजन् रम्ये कान्चन पर्वते || ६-२७-३७
मुख्यो वानर मुख्यानाम् केसरी नाम यूथपः |
O, king! This army-chief called Kesari, the chief of the commanders of monkeys, wanders in Kanchana mountain wherein there are trees which yield fruits in all seasons, inhabited by large black-bees, to which the sun circumambulates clock-wise, the mountain shining with its own colour and splendour, by whose brightness, the animals and birds ever shine with the same hue, whose table-land on the top of the mountain the great sages do not leave, wherein trees, all mango-trees laden with fruits on all sides and wherein inhabit honey-bees of excellent variety.