Every now and then a crossword will throw out the terse clue 'dawn goddess' and I grin because there are a few classic possibilities that fit common grids. The most frequent short fill is 'EOS' — three letters, Greek, clean and crossword-friendly. If the grid is longer, 'AURORA' (six letters) is the Roman counterpart and crops up often too, especially in themed puzzles or when the constructor wants a slightly more flowing entry.
Beyond those two, I keep an eye out for other traditions: 'USHAS' from Vedic mythology is five letters and sometimes appears, and Slavic myth gives us 'ZORYA' which can be clued as a dawn figure in five letters as well. So before you commit, I always check crossing letters — they instantly tell you whether the puzzle-maker wanted 3, 5, or 6 letters. Personally I love seeing 'EOS' because it feels like old-school crossword territory and it makes me smile when it slots in.