I had an Idea about Peacemain crews. No matter the crew, their jolly Roger has one symbol (of author's choice) in common. Their Jolly Rogers could be completely different, except for that one symbol, and the morganeers don't bother or don't know about the symbol. Could be something the Klabautermann does when they become strong enough, preventing the Morganeers from exploiting the Peacemain symbol.
I remember two separate fics, one based in 4th year and one based in a later year with the same basic premise.
Basically Harry gets cursed by Draco, or some other Slytherin, with a curse no one has defeated before that causes the victim to enter their mindscape and confront several animal spirits, if the spirit is defeated the cursed person gets part of their abilities, for example better eyesight from a falcon/hawk/raptor. But if the victim loses even one battle, the animals take over and they go feral, looking to mate any woman and kill any male they come across.
In the fourth year fic both Krum and Fleur know about the curse, and Fleur contacts her mother because she is scared of the ramifications of Harry losing and going feral. I can't remember if Harry was done with all of the animals in this fic by the time I stopped reading.
I remember that by the time I stopped reading the later fic Harry had defeated all the animals, but still had an animalistic instinct, so they gave him an unused classroom as a den and invited a female werewolf to help him deal with his instincts.
In both fics Draco is arrested for attend line theft for cursing the last Potter with an unbreakable curse that usually ends with the victim's death, and even Lucius is unable to help his son when he discovers which curse Draco used, and who he used it on. And both are smutty Harry/multi fics based on the tags I remember.
I only ever read fics on FFN and Ao3.