The Kingdom of Rhovanion was a realm of Men.
The Kingdom of Rhovanion was founded before the year 490 of the Third Age by a group of Northmen who were lead by a 'Prince of Rhovanion'. Rhovanion at this time referred only to the region he and his people dwelt, a strip of land east of Greenwood the Great and south of Dale and Esgaroth. It is possible that Dale and Esgaroth formed part of the Kingdom before Erebor was founded by the Dwarves of Durin's Folk.
The Men of Rhovanion fought with Gondor against the Easterlings of Rhûn, who threathened their lands, and when these were defeated by the Gondorian King Turambar in circa 550 Rhovanion became an ally of Gondor.
By 1200 the ruler of Rhovanion called himself the King of Rhovanion. In 1248 Rómendacil II of Gondor destroyed all camps of the Easterlings even beyond the Sea of Rhûn, and a strong alliance with Rhovanion was forged. Gondor retreated its borders to the Anduin, and ceded all the lands east of Anduin to Rhovanion. From this time onward the name Rhovanion referred to all of Wilderland.
The King of Rhovanion at this time was Vidugavia, and Prince Valacar of Gondor served in his army. Vidugavia's daughter Vidumavi married Valacar, and their son Vinitharya became King Eldacar of Gondor in 1432, which lead to the Kin-strife in 1437. Eldacar fled to Rhovanion, and with a Rhovanion army he reclaimed his Kingdom in 1447.
In 1636 the Great Plague devastated Rhovanion, killing more than half its people. This left Rhovanion weakened, and in 1851 the Wainriders overran and enslaved Rhovanion. For 43 years Rhovanion was enslaved, but in 1899 Rhovanion revolted, while Gondor attacked the Wainriders from the west. Rhovanion was freed, but left extremely weakened. Many Men of Rhovanion left for Gondor, where they were welcomed as distant relatives.
Some of them may have gone north to Dale and Esgaroth after 1999, when Erebor was founded.
Around 2510 a new group of Easterlings, the Balchoth, appeared, and they quickly took the old lands of Rhovanion. This was the final end of Rhovanion as an independent realm.