I was playing in my survival world when I saw a baby zombie riding a spider. How rare is this?
Per the Minecraft Wiki:
Baby zombie variants can mount spiders and cave spiders. Specifically, baby zombie variants have a 15% chance to want to be a jockey, and if they do, then upon nearing the player and before attacking, they check for one of the following and mount it: adult zombies, chickens, sheep, cows, pigs, llamas, ocelots, foxes, pandas, mooshrooms, untamed cats, untamed wolves, horses and variants, spiders and cave spiders.
Thus, assuming there is are nearby spiders (which there usually will be), then 15% of baby zombies will end up mounting spiders (or something else). Since Baby Zombies make up 5% of zombie spawns (also per Minecraft wiki), the total chance of any one zombie ending up as baby zombie riding something (which might be a spider) is 15%*5% = 0.75%