What does Flanders mare mean?

Answer 1

A Flanders Mare is not the graceful and delicate horse of a queen. In Medieval times, Belgium Horses were prized in wars for their tremendous size and strength.

These heavy, war-like draft horses were typically black and ranged from colors to bay, bay-brown to chestnut.

They were rarely lighter colors like they are today. Many accounts report that Anne was of a darker skin tone, the opposite of the beauty ideal of the time.

When Henry asked Lord Russell what he thought of his new bride he replied that 'I take her not for fair, but to be of a brown complexion.'

so in short Henry compared Anne to a Belgium Horse/mare