Flanders mare
noun
1A mare of a Flemish breed, typically strong and of large stature, and often used to pull coaches.
2 Depreciative a tall or strongly built woman (usually with the implication of unattractiveness); specifically (a nickname for) Anne of Cleves (1515–57), the fourth wife of Henry VIII.
Origin
Early 17th century; earliest use found in William Browne (1590/91–?1645), poet.