I mean seriously what is he? Speaking just biologically he makes no sense with anything we have seen so far. Creatures don't normally just grow extra limbs from nothing, evolution normally works with what it has as a base and builds off that. So did land dwelling animals evolve twice? Are there other 6 limbed creatures out there? If not what is he? Where did he come from? We need answers.
Personally, I like to leave certain things unsaid X). Keeps some of the mystery around. I call it the "Midichlorian Problem" :D
It’s pretty self explanatory if you read his lore. At least, I understand it. [sg-ezreal]
he's... magic
six limbed reptiles don't exist in nature either, no one's complaining about dragons.
He was stated to change form depending on those who came to him, so I don't think it's that strange.
Watchers are six-limbed too, iirc
In a world where this 
is from the same race as this 
... I think anything can be possible
Willump was created by Nunu. Their bio clearly shows how Willump came to look like that and it’s all because of Nunu’s imagination and his desire for a best friend named Willump. Just read their bio!
He's a yeti. On a world called Runeterra, literally meaning "Magic Earth", which is EBGRUBGBGW full of magic.
Fucking wizards, man. How is this so goddamn impossible for people to understand?
Old Willump isn’t canon, so why wouldn’t a fantasy creature have four arms?
If you read the bio it talks out Nunu approaching Willump and as he was getting closer Willumps appearance was changing to fit the child’s imagination. My guess is either yeti’s appearance are shapped by whoever is close to them imagination or The gem Willump was guarding had the ability to do so. I think the gem was a world rune and it infused itself with Nunu. That’s why the world around him is shaped by his imagination. But like the story says Willump is a yeti.
He's the last (and currently only depiction of) a modern yeti, so yeah yetis probably evolved 4 arms. They're also clearly heavily shaped and developed by magic and not just physical natural selection.
