A while ago I read this post on the boards that talked about lore that sounded BS, but was legit. It was fun to read the post and everyone's comments, so I wanted to say some off the top of my head and see if there was more absurd lore facts. If I miss any, please feel free to tell me!
(Note: Some of these will be from the previous post, and while I can't find it, just know some of these I did not know until someone else pointed it out)
1. Trundle is actually one of the weaker trolls. He became chief troll through cunning and trickery (and also managed to manipulate lissandra)
2. Olaf's main motivation for gruesome battle is the fact that a seer once said he would die a peaceful death at an old age, and as a viking, he took offence to that.
3. Ashe was once blond, and wielded an axe.
4. Skarner's people got organ harvested by Camille's family.
5. Kog maw has no malicious intent: nor does he every throw up in the lore. He just eats.
6. Kled thinks Lux is "dope".
7. Illaoi likes mangoes, and as a result, was a love interest for Gangplank.
8. Aatrox wants to die. He can't.
9. Pyke doesn't know how he came back to life. He doesn't question it.
10. Riven didn't mean to kill yasuo's master: she begged Yasuo's master to break her blade, and the master complied. The released wind magic from the broken blade ended up killing him by accident. Yikes.
11. Evelynn is a virgin.
~~12. Orianna was once a girl, but died practicing turret diving.~~ (outdated)
13. Ekko's parents are still alive, and are factory workers. He lives in a home with them.
14. Braum couldn't break open a door, so he punched through a mountain.
15. Volibear supposedly created the first river in his fight with ornn. It is implied Volibear either pissed himself or cried.
16. Ornn likes cherries.
17. Kled likes tacos (and while it isn't technically cannon, someone once pointed out that Morde can't eat tacos. F.)
18. It took all of runeterra's nations to take down Morde. Morde was just a normal dude.
19. Varus is 2/3 gay.
20. Lee sin is blind because he burned his eyes out... by sitting in a fire for days.
21. Gragus's main motivation in life is looking for the perfect drink. He is fond of true ice in his alcohol.
22. Sylas killed his master, an old farmer, and a child by accident.
23. Sivir is Azir's great great great..... great great great grand daughter.
24. Yasuo's hair is only in a ponytail because taliyah gave him some cloth.
25. Heimerdinger's turrets were actually meant to be toys for kids.
26. ~~Caitlyn isn't 18. She's younger.~~ (incorrect)
27. Xin zhao was once a fisher. He became a fighter against his will, and was undefeated.
28. Rengar was the first league champion to discover gnar.
29. Quinn has a brother named caleb.
30.~~ Tf got his powers from a voluntary lab experiment.~~ (outdated)
31. ~~Maokai didn't want to be alive. He became alive and now he's pissed.~~ (outdated)
32. Ivern was once a dick. Then he cut a tree. Now he isn't a dick.
33. Garen's in denial about lux's magic.
34. Lux is a swordswoman in demacia.
35. Akali gets her tattoos from a bird.
36. Mundo's favorite snack is toenails.
37. Galio doesn't get how sex works.
38. Jax only fights with a lamppost because he would 100% win with any other weapon. He wants to give his enemies a chance.
39. Ezreal once gave his client laxatives.
40. Sion's jaw thingy is actually the crown of jarven the ~~third.~~ first.
41. Ryze once strangled his friend.
Thats the most of what I could think of off the top of my head. If some are incorrect, I apologize. Let me know if theres more stuff I missed!