"They are now grim fighters"- who are 'they'?Why they are called so?

The cat and his companions are mentioned as 'they' in the quoted line.                                        
 On the top of the shed the cat stretches himself a few times to see if every muscle is in full working order. After then he calls his near one. After the arrival of his kindred they are not the meek creatures. They become grim fighters together. So they are called so. They do not mew for fish and milk then.