We know that the reason the 3 Marauders, James Potter, Sirius Black and Peter Pettigrew, went through the process of becoming Animagi was because their friend, Remus Lupin, was a werewolf, and they wanted to both keep him company and keep him in check.

For Sirius, the big black Dog, and James, the large Stag, that makes at least some sense. Peter, on the other hand... was a rat. Just how exactly was he going to help Lupin in any way as an animal, if he's much smaller, considerably weaker physically, and unable to cast spells? Do we know?

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    Basically calming down the Whomping Willow so the other 2 could pass through
    – Voronwé
    Jul 26 '17 at 11:06
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    You might want to clarify something here - both your post and the accepted answer seem to imply that Peters rat form was a deliberate choice, which isn't correct. Maybe rephrase to "How did he help?" rather than "How did he plan to help?"
    – DavidS
    Jul 26 '17 at 13:22
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    It does seem that animagi don't actually get to choose which animal they become. Peter did what he could, but his character/identity dictates that he is a rat.
    – AJFaraday
    Jul 26 '17 at 16:02

Being small, inconspicious, and fast enough in his rat form, Peter would be able to slip under The Whomping Willow's frenzied dance, hit the kill switch, thus allowing James and Sirius to pass through without getting their heads smashed.

‘They couldn’t keep me company as humans, so they kept me company as animals,’ said Lupin. ‘A werewolf is only a danger to people. They sneaked out of the castle every month under James’s Invisibility Cloak. They transformed, Peter, as the smallest, could slip beneath the Willow’s attacking branches and touch the knot that freezes it. They would then slip down the tunnel and join me.

Harry Potter and the Prisoner of Azkaban

He would provide psychological support as well to Remus during his transformations.

‘And they didn’t desert me at all. Instead they did something for me that would make my transformations not only bearable, but the best times of my life. They became Animagi.’

Harry Potter and the Prisoner of Azkaban

Being "animals" in a sense, Lupin was not prone to attacking the Marauders, as such they were able to accompany him during his transformations.

How did he plan on helping before becoming an Animagus?

Uncertain, as we know that you can't choose your Animagus form (see @Gallifreyan's answer). This is pure speculation, but it's likely he intended to provide psychological support regardless.

A werewolf is generally known for not being a threat to animals, according to Remus, at least. Knowing this, the Marauders become Animagi for Remus' sake, as they knew they would not be in danger in their animal forms, thus giving him psychological support during his transformations. So whatever animal Peter would have become he would still have been able to accompany Remus during his transformations.

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    Wait, so Lupin would go down first, by himself, before the tree was frozen, every Full Moon? And, occasionally, I imagine, whilst transforming? Damn. Jul 26 '17 at 11:29
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    @DisturbedNeo I believe Madam Pomfrey/Dumbledore brings him there every Full Moon. Being experienced (more so than the Marauders), they'd know how to freeze the knot using only magic, as the Golden Trio did in The Deathly Hallows
    – Voronwé
    Jul 26 '17 at 11:31
  • Both quotes from Prisoner of Azkaban; here's a book extract found on Pottermore
    – Voronwé
    Jul 26 '17 at 13:59
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    Do you mean "as they knew they would not be in danger" in your last paragraph? Jul 26 '17 at 14:53
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    @MissMonicaE I believe the answerer means that they became Animagi as otherwise they would be in danger - that is, if they tried to accompany him whilst still human. You're right though, the fragment is unclear and should probably be changed. Jul 26 '17 at 15:43

People don't choose what animal to become as an animagus. The animals are random, but carry (probably) a part of the human's character, per Pottermore:

You cannot choose your Animagus animal

If we could choose our Animagus, you can rest assured we’d all be lions, adorable puppies, or magnificent eagles. But realistically, an Animagus is closely linked to your personality. For example, the brave and rebellious James Potter is famously a stag, whereas the conniving journalist Rita Skeeter is a beetle.

We can see why Peter would become a rat... because he is a rat, really, since he

betrayed Lily and James' trust and sold them to Voldemort.

The Marauders should have taken the clue.

Peter had a use though - he was small enough to sneak past the tree which was on top of the entrance to the passage to the Shrieking Shack, and press the "button" which calmed its otherwise deadly branches.

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