Source check
Why is the dokkaebi drawn with horns?
Because that image is not Korean: it is the Japanese oni. It enters the colonial school readers with the tale Kobutori jiisan — text of 1915, illustrations of horned oni from the 1923 edition of the Joseoneo dokbon. In earlier Korean sources the dokkaebi is a man in hanbok with a horsehair hat.
The entry
This source check also appears at the foot of the entry for Dokkaebi, where the fact is set out in its context rather than in the form of a question.
What this page promises. The Method commits every entry to stating how well documented it is. This section is the uncomfortable consequence of that commitment: when something everyone repeats does not hold up to checking, it has to be said — with the name of the source and the point at which it was checked, so that anyone can repeat the check. If you find an error in these lines, report it: it ends up in the corrections log.