I found this picture on Imgur:
The Rohirrim are know [sic] for their cavalry, unlike their inspiration. Tolkien was a huge fan of Anglo-Saxen [sic] culture, he served as Professor of Anglo-Saxon. He believed that if they had cavalry, they would have been able to fend off the Nordic invasion, so he gave Rohan cavalry.
I am skeptical of this claim. As far as I know, Tolkien never used any such specific influence from Anglo-Saxon history or culture when designing the Rohirrim. He rendered the Rohirric language as Old English just to show that it was a more archaic form of Westron, represented by modern English. I don't have the books to hand, but I also remember the appendices saying the Rohirrim were only Anglo-Saxon in the general sense of being a less developed civilization living under the influence of a greater, more ancient, more advanced civilization (that being Gondor for the Rohirrim, and presumably Rome for the Anglo-Saxons). But the books themselves and the appendices don't suggest the Rohirrim were meant to be direct analogues to the Anglo-Saxons.
This doesn't sound like something Tolkien would do, or admit to doing if he had done. Is this claim actually true? Did Tolkien make the Rohirrim masters of cavalry because of some kind of "what-if" situation about the Norse invasions of England? If so, where is it documented?