Cofagrigus, personally. It's a freakin' sarcophagus, that's why!
Sarcophagi aren't necessarily evil, though. It's usually what's
inside them that could go either way. If you want to go with mummy/zombie movie clichés, sure, we could say that the dead body inside a sarcophagus is evil. On the other hand, if you're the kind of person who thinks dead body in coffin = potentially someone's grandmother,
this, or
this, then they're not so bad.
On top of that, considering Willer's looking for a Pokémon who could serve as a villain in a fanfic about pirates, having a sarcophagus be a villain might take some explaining.
theres a picture of them eating jasmine for heavens sake!
Except this never happened in canon. (In other words, canon would be the manga, the anime, the games, that kind of thing. What you're talking about is most likely another fan's drawing, which means it's not something Houndoom actually did in the officially published stuff.)
If you wanted to argue that Houndoom is basically a demon dog (horns, tail, Dark-type, general evil look to it), maybe, but then again, there
was an episode in the anime all about how one decided to protect
Togepi of all things. That and Karen, one of the characters who, in the games, was one of the few characters who actually preached all about the values of loving Pokémon, owned one herself. (That would probably imply that not all Houndoom are evil, considering one's trainer isn't evil, either, if that even makes sense.)
Then again, this argument (that certain Pokémon aren't necessarily evil thanks to the things that happened in canon) could hold true for a
lot of Pokémon. (Zoroark, for example? Was actually trying to protect Zorua in the movie.) So, I guess you could say we cycle back to viewer interpretation there.