The Carian-Albinauric theory.
Radahn got no legs, Ranni's a doll and her corpse got no legs, Rykard's a snake, Rennala doesn't walk... Things seem to line up perfectly, right?
Unfortunately. The two paintings of Rennala in-game fully depict her standing, and the Rennala that Ranni makes you fight is also standing.
Oh, and Rellana exists. She very much got legs and uses them to beat the living hell out of you. She is Rennala's younger sister and veeeery old. Messmer was described as 'like an elder brother to Radahn', Rennala is definitely older than Messmer.
Also, Ranni's corpse doesn't just have no legs, she is also missing a hand and two of her fingers are broken off, she is burnt all over. But this seems to have gone unmentioned if we are dissecting visuals.
Also, while Radahn's feet are stumps, he very much uses and moves his legs during the fight, he can walk fine on those stumps. Check image 7.
...Also: Rykard's painting shows him very much standing.