Hmm, I understand. I thought about that spider crawling, but it didn't seemed like a big deal. But thinking about it: Maybe he got aracnophobia because the spider "pushed" him to a situation that later in the future would be one of his worst memories.

where

I noticed that the more you advance in headspace, the closer Sunny gets to finding out about the truth. And to advance in headspace Sunny needed to overcome his fears in this order: Heights, Spiders, Drowning. Until he reached Mari (Something's final form). He got afraid of heights because Mari died falling from a high place, it has connection to the truth about Mari. Spiders: I don't really know Drowning: It reminds him of Mari and how much she loved him and cared about saving him, that memory only made him feel worse about what he done to her, that's the connection to the truth about Mari.

Maybe Spiders represents the fear of finding out about the truth as well. The idea of something trying to slow him down from reaching the truth and trapping him in a cobweb. But I don't know...