SB 10.14.20

sureṣv ṛṣiṣv īśa tathaiva nṛṣv api
 tiryakṣu yādaḥsv api te ’janasya
janmāsatāṁ durmada-nigrahāya
 prabho vidhātaḥ sad-anugrahāya ca
Synonyms: 
sureṣu — among the demigods; ṛṣiṣu — among the great sages; īśa — O Lord; tathā — as well as; eva — indeed; nṛṣu — among the human beings; api — and; tiryakṣu — among animals; yādaḥsu — among aquatics; api — also; te — of You; ajanasya — who never takes material birth; janma — the birth; asatām — of the nondevotees; durmada — the false pride; nigrahāya — for the purpose of subduing; prabho — O master; vidhātaḥ — O creator; sat — to the faithful devotees; anugrahāya — for the purpose of showing mercy; ca — and.
Translation: 
O Lord, O supreme creator and master, You have no material birth, yet to defeat the false pride of the faithless demons and show mercy to Your saintly devotees, You take birth among the demigods, sages, human beings, animals and even the aquatics.
Purport: 

Among the demigods Lord Kṛṣṇa appears in such forms as Vāmanadeva, among the sages as Paraśurāma, among human beings as Lord Kṛṣṇa Himself and as Lord Rāmacandra, and among animals as the boar incarnation. Lord Kṛṣṇa appears among the aquatics as Matsya, the gigantic fish. Indeed, the plenary expansions of the Supreme Personality of Godhead are innumerable, as the Lord relentlessly comes down within the universes to smash the false pride of the atheists and show mercy to the saintly devotees.

In another sense, the Lord never appears, since He exists eternally. His appearance is like that of the sun, which is always present in the sky but which periodically appears to our vision.