The two brothers who by reputation are remembered as founders of the Anglo-Saxon Kingdom of Kent.

The tale of Vortigern hiring three boat loads of Saxon foederati and settling them in Kent can reasonably be accepted as historical truth. This comes from Gildas, whose De Excidio Britanniae. is the closest thing we have to a contemporary source for this period.

It is Bede in his Historia who first names these individuals,

The first commanders of the strangers are said to have been two brothers, Hengist and Horsa. Of whom Horsa, being afterwards slain in battle by the Britons, was buried in the eastern parts of Kent, where a monument bearing his name, is still in existence. (1)
and even provides us with a geneology, stating that they were the sons of Whitgils, son of Wecta, son of Woden.

Nennius in the Historia Brittonum then expands on this with further detail including the tale of how Hengist, acting under the pretence of ratifying a treaty, treachorously slays three hundred of Vortigern's nobles but spares Vortigern himself on account of his marriage to Hengist's daughter. Vortigern them purchases his freedom by delivering up the,

three provinces of East, South, and Middle Sex, besides other districts at the option of his betrayers

By the time we get to Geoffrey of Monmouth the whole thing has been turned into a mini epic with a complete cast of supporting charcters.

For the historical record I would note that Bede states that they are said to have been, a clear indication that he was simply reporting on an oral tradition and not something he considered to be hard fact.

I understand that Hengist and Horsa are Anglo-Saxon for stallion and mare (2), and whereas I can quite believe that an Anglo-Saxon warrior lord might well be called "stallion", the notion of one named after a female horse seems quite comical.

Some suggest on this basis that the names Hengist and Horsa refer to one and the same person, but the names sound to me as if they hark back to some kind of creation myth, a Germanic pagan Adam and Eve, who become, with the transposition of gender, two brothers, and take part in different sort of creation myth, one that would be more acceptable to a now Christian audience of Anglo-Saxons.

Which is one way of saying that the general consense is these days is that Hengist and Horsa were the products of the imagination.

(1)A statement based on the misreading of a Roman monument bearing the Latin inscription CAHORS. Fifth century Jutes didn't go in for monuments in Latin.

(2)This is what I understand, I would not swear to it, given that my knowledge of early Anglo-Saxon is extremely limited.