Couple of different things.
The ancient Greek scholia on Homer's Odyssey give several different accounts of the origin of Kharybdis. In one she was a daughter of Pontos (Sea) and Gaia (Earth) who laid siege to the land with her waves. Zeus, in anger, captured and chained her to the sea-bed. In another tale, she was a voracious women who stole the cattle of Herakles. For this reason Zeus cast her into the sea with the strike of a thunderbolt.
So according to Suidas, she could have
been a monster already, who didn't like Zeus, or
been a woman who stole cattle.
Also, she was speculated to be the goddess of tides.
KHARYBDIS (or Charybdis) was a sea monster or goddess whose gigantic whirlpool swirled in the straits of Messina, opposite the cliffs of the monster Skylla. She was probably the goddess of the tides, with her triple drawing-expulsion, mentioned by Homer, representing the three high-low tides of the day. Aristotle also clearly identified her with the tides.
So before Skyla, even, she was the goddess of tides, because, well, the tides always happened, right?