I haven't looked too much into this, but when mobs (possibly entities in general) dismount another entity, it seems they become invisible, and you need to restart Minecraft for them to become visible again. I have experienced this twice: Once when I broke the minecart a villager was riding, and the villager became invisible, and another time where the spider of a spider jockey died first, and the skeleton became invisible; it kept shooting arrows at me, without me being able to see it.