The beastmen (マンビースト Manbeast) are a race of hybrid animal-humans that mutated over the thirty years since the Golden Sun event at the end of Golden Sun: The Lost Age. Most known beastmen live within the country of Morgal, located in the northeast corner of Angara.
In the aftermath of the Golden Sun event above Mt. Aleph, various humans began to grow fur and fangs, while various wild animals began to grow in size, stand on two legs, become sapient and gained the ability to talk. While beastmen were initially located throughout Weyard, most migrated to the northeast section of Angara, though no real reason brought them together. Some beastmen are said to have unique powers, although the only beastman whose abilities are seen in detail is Sveta. In any case, the beastmen appear to have several Psynergies that are unique to them, such as Slap.
At some point, the beastmen formed their own country, led by their own Volechek's father. This country was later conquered by Sana, and oppressed. This changed when Volechek, son of the former king, rebelled against Sana. In a violent uprising in which the previous Sanan Emperor was killed, successfully reestablished Morgal as an independent county with Belinsk as its capital. Belinsk eventually became haven of sorts for the beastmen across Angara.
Many beastmen were killed as a result of the Grave Eclipse, due to Belinsk being the location where the eclipse was triggered. Once the eclipse ended however, the remaining beastmen of Belinsk were inundated with light from the Apollo Lens. As a result, the Beastmen's fur turned golden and they apparently became Adepts with the ability to manipulate light. In game dialogue seems to suggest that this change was limited to beastmen who had previously been human (as opposed to those who mutated from animals) though no reason is given for why this would be the case.
Even though their emergence is a relatively recent phenomenon, beastmen are known to have existed during the time prior to the sealing of Alchemy. They are known to have worked alongside the Jenei as indicated by the Umbra Gear being designed specifically for beastmen but little else is known about them, including why they vanished.