Yemen (North) 1971
The first miniature sheets (or souvenir sheets) were issued before W.W. II, usually with a surtax for raising funds for a charity rather than for sale to collectors. They became more of a collectors item with pictorial issues around 1970.
In 1971 the first miniature sheet with a prehistoric animal was issued by North Yemen. Fujeira, Manama, and the Maldives (all 1972) and other countries followed quickly.