ok so ive been sitting on this for a Hot Min actually cuz i couldn't think of a "good" answer
surface level fear: rust/rusty metal, like the stuff he's made of is pretty rust proof as a rule but he still has nightmares about tetanus and falling apart
repressed fear: i don't think dysphoria counts as a fear but thats the closest thing he's got i guess? "if im not cute/soft/pretty/sexy people won't care about me and i'll be all alone again" type shit
deep dark fear: becoming a monster, like castlevania dracula tier "i enjoy hurting innocent people" type shit, he's petty and mean on occasion but he usually won't hurt anything or anybody that isn't explicitly asking for it (ie consenting to a fight or picking on him/someone he loves). He's so concerned that he's becoming something awful or that he's already awful and everyone is starting to see past his 7 proxies. He has this weird fucked up quirk where the more he loves someone the more he wants to turn them into a beast or creature, adding claws or sharp teeth or tails, and he's not sure why he does that but he's concerned that he's gonna take away someone's autonomy and fuck them over like his caretakers fucked him over when he was a kid (ship of theseus)