Benjamin Malvern is Mutt's father and the owner of the largest stable on Thisby. But he's barely a father, and he's motivated by a love for money instead of a love for horses. He's a man of rich tastes, drinking his tea with "butter, milk, and salt" (16.31), which sounds super gross. And when he tells Puck, "I'm not a generous person" (16.68), he's not kidding. He has no qualms about evicting two orphans from their homes, and he drives a hard bargain when Sean wants to buy a horse that he's all but raised.
Malvern tells Sean that he's a self-made man. He started off poor, but he caught himself a horse and has traded it up step-by-step to his own private fortune, like that guy who traded a paper clip for a house. We want to believe his success story, but he then goes on to talk about a solid-gold horse and a blindfold made from a virgin's dress, so he could just be full of it.
However he made his fortune, it's no excuse for the man to be so cruel. We painted Mutt with the antagonist brush, but Mutt's behavior probably wouldn't be so reprehensible if his father would just toss him some love now and then.