Well, not really a compendium but a list. "Compendium" sounds cooler and smarter. Going to leave it for those who want to use it. I'll keep it updated for anyone who makes and notify me new onion head icons. If it's NSFW, that may or may not go on the list. PM me if you want to have your plz account put on the list.
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ONION TOU/HEAD ICONS
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Here is a list of of account creators and the plz accounts they made. Note that the avatars are either made by Onion Head creator Ethan Liu or Onion Head fans.
ONIONHEADS:
Onion Head plz accounts made by me:
~vomitplz (https://www.deviantart.com/vomitplz):iconh