Feral Sanguinius from Mahaute's Planet-Swap-AU-fanfic.
He was raised by predatory monsterbirds and is clueless about civilisation.
It's a very good fic and I'm so excited for the next chapters (I already got some teasers. Like Mobster-Mournival, Diplomat-Sevatar and such. Sounds very great!)
Please read it, because it's very good and Fanfiction is the best thing the Warhammer-fandom has to offer!
https://archiveofourown.org/works/41683488