A magistrate has jailed a teenager for killing a cat, describing the attack as sickening.

In April 2008, the elderly cat was stolen by a group of children and hanged from a bridge at Craigieburn, north of Melbourne.

Two boys, both aged 15 at the time, were charged with theft and aggravated cruelty to an animal.

Children's Court Magistrate John Doherty said he was outraged, mortified and sickened by how the boys killed a defenceless family pet.

He sentenced one boy to four months in a youth justice centre.

The second boy was given a six-month probation order.

The court heard he was genuinely remorseful.