First Three "Avatars" of Vishnu.

The first Avatar was Matsya or fish. It had a double purpose, to save Manu, the progenitor of the human race (Adam?) from a deluge (Noah's Ark?) & to recover the Vedas from the demon Hayagriva. It can be surmised that reading & writing were invented by this time to preserve the Vedas, which were heretofore orally transmitted from one generation to the next.

The second Avatar was Koorma or tortoise, who acted as the pedestal for the churning stick in the Amrita Manthan episode, when the Devas & Danavas fought for the elixir of immortality.

The third Avatar was the Varaha or boar whose function was to kill

Hiranyaksha, a demon, who had kidnapped Bhoodevi or Mother Earth.

It may be noted that the above three Avatars, along with the well-known Narasimha Avatar (fourth in the series), who incarnated to save his devotee Prahlada (incidentally a Danava by birth, who was harassed by his father, Hiranyakashipu), were in the Satya Yuga or Righteous Age, when Dharma was supposedly fully stable on all four legs.

A very popular deity, who does not fit into the Avatar list is Lord Venkateswara (of Tirumala).