What is the criteria to consider anyone as an avatar (reincarnation)? Sri Krishna is not considered an avatar in many religions other than Hinduism.
In Srimad Bhagavatam it is said, the divine has descended in the swans, in the fire, in the trees, in all animals. The entire creation is nothing but the manifestation of divinity, and wherever you get this message (referring to knowledge) from, you call that as an avatar.
Sage Dattatreya has noted 24 different forms of life. He received knowledge from a swan and said that the swan was a form of Vishnu.
You should know that God is present in every particle of the universe. The cheapest commodity available is the divinity! It is present in the Earth, Water, Fire, Air and Ether. Everything is made up of one vibration and it can be called by any name, Brahman, divinity, God. So, when divinity shines through one who gives knowledge and can kindle love, then one considers that as an avatar.