"Oh - sorry, Neville! ... I forgot - it was the custard..."
"... creams we hexed -"
-- Fred and George Weasley (GF21)
A temporary hex to change someone into a giant canary. After a few moments, the Transfigured person molts back into themselves.
References from the canon
Fred and George placed this hex on seemingly innocent custard creams so that whomever ate one was changed temporarily into a huge canary. Neville ate one of these Canary Creams, much to the amusement of everyone in the Gryffindor common room (GF21).
Jinxes and hexes