'Crookshanks doesn't understand it's wrong!' said Hermione, her voice shaking. 'All cats chase rats, Ron!'
'There's something funny about that animal!' said Ron, who was trying to persuade a frantically wiggling Scabbers back into his pocket. 'It heard me say that Scabbers was in my bag!'
'Oh, what rubbish,' said Hermione impatiently. 'Crookshanks could smell him, Ron, how else d'you think —'
'That cat's got it in for Scabbers!' said Ron, ignoring the people around him, who were starting to giggle. 'And Scabbers was here first, and he's ill!'
Ron marched through the common room and out of sight up the stairs to the boys' dormitories.
I am not sure what "it" refers to in "That cat's got it in for Scabbers!". Is it referring to the cat itself or something else?
-- From Harry Potter - and the Prisoner of Azkaban, Chapter 8.