A haiku—an English-language haiku, that is—need not, and probably should not, have seventeen syllables. ‘Syllable’ is a misunderstanding of the Japanese ‘on,’ a unit that is often shorter than a syllable. If we insist on the five-seven-five syllable structure, our poem may well end up longer than a haiku in Japanese. Twelve or thirteen syllables will tend to be closer in length to a Japanese-language haiku.
A ‘true’ haiku should contain references to nature and a season. It should not include any explicit metaphors. If it does not fit these criteria, it is some related form, such as senryu or zappai. All these descend from haikai, which in turn derived from the linked poetry known as renga (which also is ancestral to the tanka form).
I rarely pen a haiku in the strict sense. Senryu and related forms, yes, though I may or may not use the seventeen syllable structure. Definitely not more than seventeen!
