"The sagas tell how the very next day Humongously Hotshot, dressed all in black, sailed up the Gorge of the Thunderbolt of Thor and into the heartland of UG's territories, right under the nose of the sleeping Screech Dragons and somehow escaping the notice of the swooping Raptortongues."
How to Seize a Dragon's Jewel

Screech Dragons are mentioned once in How to Seize a Dragon's Jewel at the end, when discussing how Humongously Hotshot kidnapped Princess Tantrum. Because the Screech Dragon is mentioned as living in the Gorge, they may be classified as Mountain Dragons. Raptortongues are also Mountain Dragons.

It is possible they are loud and vocal dragons, due to the 'Screech' in their names. Or perhaps they are sometimes utilized as guard dragons, since the book notes Hotshot had to sail by them stealthily to get to Tantrum.