While searching for the stray jockey mob, I tested out in creative to see if it exists, apparently in almost all minecraft sources it says that stray jockeys exists and can spawn naturally, but upon testing it does not in the current version 

How to duplicate:

1. In a snowy biome set up 2 command blocks, both needs redstone and on repeat, one command block executes the command to summon spiders, and the other to kill the spiders as the attached image shows
2. Then turn of the command blocks

Observation: no matter how long you wait not a single stray will come out of the spiders, thus Stray jockeys and as well as wither jockeys does not spawn naturally and by commands.