Finland and the other countries surrounding the Gulf of Finland were the home region for the so-called "yellow pony". A later ethnologist, Kustaa Vilkuna (1902–1980) supported this view, proposing that an "Estonian-Finno-Karelian pony" descended from a small forest horse previously widespread in the lands surrounding the Gulf of Finland. Earliest horse equipment (bits) found in Finnish graves date from the Finnish Middle Iron Age, beginning from circa 400 CE. Breeds considered to descend from the same early types as the Finnhorse include the Estonian Native horse, the Norwegian Nordlandshest/Lyngshest, the Swedish Gotland Russ, the Mezen horse from the region of Archangelsk, Russia, and the Lithuanian Žemaitukas. At some point in their history, not clearly documented, horses bred in the western regions crossbred with horses that originated south of the Gulf of Finland. This made the western Finnish horse type larger and better suited to farming and forestry work. The characteristics of the original western Finnish type prevailed, however, even though influenced by outside blood and traces of outside influence could be detected for a long time. Later, this mixed type was further crossbred with larger horses from Central Europe during the Middle Ages. Foreign horses were also brought to Finland during military campaigns, and additional animals were imported to manor houses for driving. The crossbreed offspring of Central European and Finnish horses were larger than their Finnish parents, and even more suited for agricultural work. The earliest known documentation of Finnish trade in horses, both as imports and exports, dates to 1299, when Pope Gregory IX sent a letter of reprimand to the merchants of Gotland, who were selling horses to the non-Christianized Finns. Apparently the Finns succeeded in improving their horse population, as the predominant form of Finnish trade in horses eventually shifted from imports to exports. A Russian chronicle from 1338 mentions "Tamma-Karjala" ("Karelia of the Mares"), presumably denoting a place of good horse breeding. As early as in 1347, King Magnus IV saw it necessary to put limits to the horse exports from Karelia to Russia. Later, the 16th century writer Olaus Magnus mentioned the high quality of the horses used by the early Finns; in the 1520s, Gustav Vasa found the Finns exporting horses by the shipload to Lübeck, and strictly prohibited such trading, banning the sale of horses under the age of 7 years. Organised breeding The earliest document noting the importation of outside horses