Vishnu took the form of a fish to save the sacred Vedas from the demon Hayagriva and to rescue the sage Manu and the seven sages during the great deluge.
Vishnu incarnated as a boar to rescue the Earth (personified as the goddess Bhudevi) from the demon Hiranyaksha, who had dragged it to the bottom of the cosmic ocean.
Vishnu took the form of a dwarf Brahmin to subdue the demon king Bali. By asking for three paces of land, he then expanded to cover the entire universe in three steps, thus reclaiming it from Bali.
Vishnu incarnated as a Brahmin warrior who wielded an axe to rid the world of corrupt and oppressive Kshatriya (warrior) rulers.
In this avatar, Vishnu was born as Prince Rama, the hero of the epic Ramayana. He exemplified the ideals of dharma, duty, and righteousness, and he defeated the demon king Ravana.
Vishnu's incarnation as Krishna is celebrated in the Mahabharata and the Bhagavad Gita. Krishna's life and teachings cover a wide array of divine activities, including his role as a charioteer and guide to Arjuna in the Kurukshetra War.
Some traditions include Buddha as an avatar of Vishnu, who incarnated to teach compassion and non-violence, challenging the ritualistic practices of the time.
The final avatar of Vishnu is yet to appear. Kalki is prophesied to come at the end of the current age of Kali Yuga, riding a white horse and wielding a sword to eradicate evil and restore dharma.