So ever since TFT came out I wondered if Little Legends could somehow be a part of the canon universe...
Let's just explore the possibilities!
The runespirit seems like quite a curious "thing" as it is essentially animated stone. We do know that animated stone exists in the universe of LoL and a prime example of that are krugs. So the possiblity is there and judging by the look of the runespirit it seems like it's home may be Ionia. Now there is also another example of animated stone and that woulde be Daisy herself. If we keep Daisy in mind as well as the Krugs and the (not necessarily canon) stone golems from the Summoners Rift, the Runespirit may quite possibly be canon.
This creature looks like a mix of a cat/lion and a ram. I really can not see the furyhorn being canon, however there is a slim chance of it being a freljordian "being" due to the ram connection(and Ornn).
Featherknights may or may not be a part of the freljordian region due to their general appearance. We definitely know that the creature is a part of the "Snowdown Showdown" universe. Anything else above this point would be a very vague assumption.
Well regarding the Molediver, my theory is that this species would live between Demacia and Noxus as the territory seems very broad and "earthy" and with that a perfect place for burrowing. Their physical appearance with their strong forelimbs and rather small eyes makes them quite realistic and with that quite possibly canon.
Right away a "creature" like that seems to be a part of the Shadow Isles. The ghostly appearance plus the chain really give it a creepy and lonely feel. Now I myself have not quite seen such a ghost on any pictures on the universe page or the map however, there have been similar "things" which makes the Hauntling more believable.
The Silverwing could potentially be a young demacian Raptor. Looking at the Raptors when they are grown, they have dark-grey feathers with a huge beak. So the young Raptor, or rather the demacian Silverwing would have blue-golden feathers that they would lose when grown. This is probably the most believable instance of a Little Legends being canon.
To me personally, I would say the most believable candidates for being real/canon beings are the Molediver and the Silverwing given their appearance and overall believability.