Class 1A went to a place where specimens of dragons were sold and there was only one that was very strange. Aizawa knew that there they maltreated the dragons to "tame" them (but they only hurt them).

That dragon was a 'Ymir', a type of white dragon that is from Japanese mythologies. The Ymirs live in packs, the females had longer horns and spots under the eyes, the males were smaller and larger

Aizawa "There it is.. A Ymir.."

Deku "Why do we come to see a dragon, Aizawa?"

Aizawa "Because we going to get them out of here." he says and just looks at you