A traditional haiku is 5 syllables, 7 syllables, 5 syllables — as you already knew.
A Milk Carton Haiku was created by the editors of NCiR’ and is 4 syllables, 5 syllables, 4 syllables. The pictures of the missing syllables can be seen on Milk Cartons, and none have ever been found. Sometimes, the topic of a Milk Carton Haiku is something over which milk can be poured, like cereal!
A Nonsense Haiku (sponsored by Haiku Hub) uses the traditional form, but the poet’s meaning is likely to be lost in the nonsense! 😜