First children of Gaia and Uranus:

Cottus, Briareus (Aegaeon), Gyges (Gyes).

Each of these three had 100 arms and 50 heads. That's why when they add up, they have 300 sleeves.

To these creatures

GIANT

They were called giants, but they were very different from the later giants in mythology.

The Hundred Arms had tremendous strength. They could throw huge boulders and raze mountains to the ground. He was not without fear of such power, including their father, Uranus.

You are the father, you rule the universe, you are afraid of your children...

Out of fear he imprisoned them in Tartarus in the underworld.