In chapter 43 of Anna Sewell's Black Beauty, the lady at the hospital mentions "Mary" several times. It seems like Mary is Jerry's wife:
“Well, Barker,” she said, “it would be a great pity that you should seriously risk your health in this work, not only for your own but for Mary's and the children's sake; there are many places where good drivers or good grooms are wanted, and if ever you think you ought to give up this cab work let me know.” Then sending some kind messages to Mary she put something into his hand, saying, “There is five shillings each for the two children; Mary will know how to spend it.”
But Jerry's wife is Polly:
Jeremiah Barker was my new master's name, but as every one called him Jerry, I shall do the same. Polly, his wife, was just as good a match as a man could have.
So who is Mary? She seems to be mentioned five times in that chapter, but nowhere else.