Minecraft 1.10 saw the addition of Strays to the game. Strays were originally variants of skeletons that only spawned in frozen, ice or snowy biomes. However, with the release of Minecraft 1.17, mechanics regarding Strays have changed ever so slightly.
Rather than being a variant of skeletons that spawn on their own, skeletons are now able to be converted into Strays as well as spawn as they regularly do.
Strays in Minecraft
Conversion of Skeleton to Stray
The Stray will spawn at full health, and if the skeleton is holding an item or wearing a piece of armor, it will stay with them when they turn into a Stray.