Many of the objects in tomb mounds found in Japan were also found in Korea like armor, sword, horse-related objects ( noting that Japan didn't have horses ).
Because both countries shared Han cultural-origin of Lelang
Pai, Hyung Il. "Culture Contact and Culture Change: The Korean Peninsula and its Relations with the
Han Dynasty Commandery of Lelang." World Archaeology 23:3 (February 1992): 306-319.
Lelang Commandery - Wikipedia
One of the most controversial topics in Korean archaeology and history concerns the Han Lelang commanderies that were established in the Korean peninsula during the Han dynasty in 108 BC and lasted for 400 years.
In the traditional view, the importance of the Lelang in Korean history was seen in its role as the common enemy, at the time when Korea first experienced colonial rule.Such sentiment is so strong that there are actually some Korean ultranationalists who even deny the very existence of the Lelang commandery.
This view has been further complicated by the fact that some of the earliest archaeological work on Lelang was initiated by the Japanese around the time of the First World War. Back then, the ulterior motives of territorial claims by the Japanese Government General's Office of Korea over the Korean peninsula and Manchuria made some Japanese Lelang scholars claim that Han Lelang culture in Korea was a purely Han Chinese phenomenon, with no native Korean variants and forms.
But what really were the relations between the Korean Peninsula and the Han Dynasty Commandery of Lelang ? Hyung Il Pai (1992) presented a more balanced theoretical view that may help reconcile the nationalistic view of contemporary Korean scholars with pre-war Japanese colonial interpretations of Han Lelang's position in Korean prehistory. Basically,she argued that social and regional differentiation in the Korean peninsula were not possible without initial Han contact. Before the Lelang period, regional differences find expression ony in terms of minor variation in pottery styles. In contrast, third-century texts of the Weizhi (300 years after initial Han contact) reveal the existence of various guo 国 (or tribal kingdoms) such as Puyo, Koguryo, Okcho, Eastern Ye and Samhan. They are recorded as having distinctively different social organizations, subsistence systems, customs, and rituals.
The most important "traits of civilization" such as iron technology, writing, gold craftsmanship, and intesive rice agriculture, were derived from Han Lelang culture. Such widespread distribution of ideas and technology would not have been possible without the elite distribution network of seals, bronze mirrors, and luxury Han goods which stimlulated the initial exchange network.
According to Pai (1992), once
this network was established in the core areas of Lelang, it quickly spread to all other parts of Korea and into southern-western Japan, forming the "Lelang Interaction Sphere" in Korean prehistory (that included Koguryo, Wa of Japan, and Samhan - Chinhan, Pyonhan, and Mahan)".Without this initial phase, the second stage of the interaction sphere that comprised Koguryo, Paekche, Silla, and Kofun Japan, would not have been possible. Extensive trade and diplomatic activities were heightened and reinforced by competition and warfare with Yamato Japan and the three kingdoms. These states shared common features in palatial architecture, in the spread of Buddhism and associated sculpture, as well as in gold artwork and jewellery.
References:
Pai, Hyung Il. "Lelang and the 'Interaction Sphere': An Alternative Approach to Korean State Formation." Archaeological Review from Cambridge 8:1 (1989): 64-75.
http://www.eastasian.ucsb.edu/content/people_pai.html
J. Edward Kidder, Jr. (who released a 400+ page book on the history, archaeology, and mythology surrounding Himiko and the location of Yamatai this Spring)
covers the history of horses in Japan in his article "The Archaeology of the Early Horse-Riders in Japan". You can read it in "The Transactions of the Asiatic Society of Japan".
He also provided some great evidence that discredits a few pillars of Egami's horseriders theory.Of course,every theory has aspects that reflect cultural/social/historical currents.
I recommend you guys checking out his article. I'll only lay out a few points he made in this post.
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The Kiso horse is indigenous to Japan, yet it's presently endangered. The horse that spread throughout the archipelago was imported from Korea.
-Horses began to be ridden widely in the 5th century.
-It was expected that aristocrats know how to ride horses.
-The native horses were from the Late Jomon period (1000 B.C.-300 B.C.)
-He gives 114.5 cm as an average height for the indigenous horses (measured up to the "withers")
-Yayoi horses are, on average, 132 cm (again, "withers")
-Jomon and Yayoi (300 B.C.-250 A.D.) sites don't imply the eating or sacrifice of horses. I believe Farris mentioned they were beasts of burden, however they apparently weren't ridden, yet.
-After the Yayoi period, horses began to be used in religious rituals. They were sacrificed. The Taika Reforms of the 7th century prohibited the sacrificing of horses (among other mourning practices after one's lord/leader had died).
-Horse sacrifice is actually a bit debated, as Kidder wonders why the Japanese would sacrifice the few horses that lived on the islands during the first years of the Kofun period.
-The use of horse
haniwa around kofun seems to imply that the Japanese believed that horses were mediums or intouch with the spiritual world. 8th century rituals involving horse figures attest to this.
There's a lot more information in the short article. If you can get your hands on it, it'd go nicely with Walter Edwards rebuttal of Egami's theory. Both are from an archaeological standpoint.