A branch of Arachnophobia, people affected with this are scared of Minecraft spiders (this may not include real spiders). This may be due to the fact that spiders in Minecraft are practically invisible at night when they are aggressive towards the player. And their small height of one block, the subsequent ability to crawl into narrow tunnels and the ability to climb walls only add fuel to the fire of fear. Plus, there is a cave variety of spider that appears in cobweb-filled mining tunnels, even smaller and able to poison the enemy. Because of it, there is also a fear of entering caves.
The best way to combat Cubrancharachnophobia is to teach the fearful one how to fight spiders. It also wouldn't hurt to have a weapon enchanted with the Arthropod Scourge to make the phobes kill them much faster.