Professor McGonagall was one of just seven Animagi registered in the last century, according to Hermione, who told us that the Ministry’s Improper Use of Magic Office kept close tabs on wizards with this ability.

But the ability to transform into animals was sometimes a scary business. A ‘near riot’ was caused by a group of 14-year-olds from the Ugandan wizarding school Uagadou at the International Symposium of Animagi. Their synchronised transformation into elephants and cheetahs intimidated some of the older and more experienced witches and wizards.

It sounds like the number of Animagi might have increased in the wizarding world, and even during Harry’s time at Hogwarts it was clear there were at least four unregistered ones…

Rita Skeeter

The sensationalist Daily Prophet reporter whom Dumbledore had banned from Hogwarts grounds managed somehow to wheedle her way back in by changing into her illegal Animagus form: that of a nasty little beetle. Appropriate enough for a journalist who scuttled merrily over the misfortune of others and fed on trash, you might say.

The punishment for being an unregistered Animagus was time in Azkaban so, when clever-clogs Hermione discovered Rita’s secret, the threat of exposure was enough leverage to make her stop writing those poisonous articles.

Her Quick-Quotes Quill was next used to write a balanced interview for The Quibbler (much to her chagrin), giving Harry’s account of Voldemort’s return. Naturally, she protested but Hermione’s jokey suggestion that the Daily Prophet might pay well for an insider’s account of Azkaban was enough to get Rita on side.

James Potter

Harry’s father James and godfather Sirius Black worked for three years during their time at Hogwarts to become Animagi to keep their friend Remus Lupin company during the full moon. Werewolves might be dangerous to humans, but they don’t attack other animals; so Lupin’s friends found a way to stay together as a kind of pack.

James’s Animagus form was that of a stag, which earned him his nickname, Prongs. Strikingly, Harry’s Patronus was a stag and his mother Lily’s was a doe, a female deer, showing that the family’s characters were in harmony and formed part of the same animal group.

Sirius Black

It wasn’t until their fifth year that three of the Marauders became Animagi – the process was notoriously difficult, and could go horribly wrong.

Luckily, Sirius and James were the among the brightest students in their year and perfected the magic needed so that they could keep their Lupin company during his monthly transformations. Sirius’s Animagus form was that of a big shaggy-haired black dog, hence his nickname – Padfoot.

Even Dumbledore wasn’t aware that Sirius and James had learned this skill. When Sirius was accused of betraying the Potters to Voldemort, nobody left alive except Lupin knew he could pass himself off as a dog (well, nobody except Peter, who was too busy hiding as a rat to tell anyone).

Having this ability certainly paid off down the line, as it allowed Sirius to escape from Azkaban. It also meant he was able to gain access to Hogwarts and Harry, once he learned the truth.

Harry might have mistaken Sirius for a Grim – an unlucky omen – but discovering he had a godfather who was both alive and who had bought him a Firebolt broomstick was perhaps one of the least grim moments of his life.

Peter Pettigrew

Perhaps it should have been obvious from the form he took that Peter Pettigrew was at risk of betraying his friends. The rat who Ron lovingly cared for for 12 years was identifiable as Wormtail from his missing finger.

Up until his exposure it had been believed that Pettigrew had died soon after the Potters were murdered and that the biggest part that remained of him had been that finger. As it turned out, Wormtail cut that finger off himself and staged his phoney death to implicate Sirius instead.

Surviving on bits of fluff and jelly beans in Ron’s pocket as Scabbers, Pettigrew bided his time. Sirius and Lupin caught up with Peter eventually, but in his rodent form he slipped through their grasp and back to the Dark Lord’s side.