Minecraft snapshot 25w05a introduced cow variants, and in addition mirrored the leg texture. However, pig models were not given the same change.
Attached are two screenshots:
One in game of the cow and pig temperate variants side by side, showing that the pig model’s legs are not mirrored.
And a screenshot of the Minecraft Live 2025 Trailer, showing that in promotional material the pig model’s legs are mirrored.
Other mobs that do not have their leg textures mirrored include:
Cats
Goats
Llamas and Trader Llamas
Ocelots
Pandas
Polar Bears
Sheep
Wolves
Armadillos, Camels, Donkeys, Foxes, Horses, Mules, Skeleton Horses, and Striders have their leg textures properly mirrored.
A few mobs such as Rabbits and Sniffers do not have their leg textures mirrored, however their leg textures seem to be intentionally unique.
Still confirmed in 1.21.5 Pre-Release 1