croar

The specific verb for the sound frogs (and toads) make — croaking. It's a regular -ar verb and almost always used literally about frogs in ponds or nature descriptions; there's no common figurative extension the way graznar (crows) sometimes gets used for a harsh human voice.

• Las ranas croaban toda la noche junto al estanque. — The frogs croaked all night by the pond.
• Se oía croar a las ranas desde la ventana. — You could hear the frogs croaking from the window.
• Cuando llueve, las ranas croan con más fuerza. — When it rains, the frogs croak more loudly.

It's specific to frogs/toads — don't reach for it to describe other animal sounds (birds use cantar or piar, crows use graznar); Spanish, like English, keeps its animal-sound verbs separate by species.