Definition of "casque"

He beheld his child dashed to pieces, and almost buried under an enormous helmet, an hundred times more large than any casque ever made for human being, and shaded with a proportionable quantity of black feathers.

1764, Horace Walpole, The Castle of Otranto, section I

Helmeted Hornbill, Rhinoplax vigil (CR): Restricted to the Thai-Malay Peninsula, Sumatra and Borneo, this is the only hornbill species whose casque is solid keratin and therefore carvable.

2015, James Eaton et al., “Trade-driven extinctions and near-extinctions of avian taxa in Sundaic Indonesia”, in Forktail, page 2, column 2