A Little Fire Dragon? No More!

He crosses into another world in the body of a small fire dragon. A girl summons him there. She reaches into him, pulls out his dragon core, and feeds it to a bear, then kicks him onto a rubbish heap and leaves him there. He is close to dead when the most popular girl at the academy finds him and saves him by binding him to a contract. After that he swallows a toad whole and evolves on the spot, and he keeps growing from there. When a beast tide comes and everyone else runs, he goes after the beasts and eats them. The ones who took his core see him do it and go down on their knees, and one of them asks whether this is really the F-rank trash dragon they threw out.