Warlock was Millicent's three-legged, ill-tempered cat; he'd lost a leg to Monty.
"It isn't uncommon for the broken ones to turn violent."
Born a Squib, Monty's jealous reaction to the "spoilt" nine-year-old Millicent's birthday kitten had proved the last straw for the Bulstrodes, who'd summoned a Healer to remove their son.
At the time, Monty had scratched and bitten and yowled, and so had Millicent—but despite the adults' threats, she'd not let go of Warlock.
"And I won't let go of you now," she whispered to her growling boy, as she and the other Slytherins were banished from Hogwarts.