I don't like it, but the design has more to it than those who hate it want to admit, it's really clever.
-First, it doesn't look like a monster because it isn't one, it's a regular sword, but haunted, so, no face, no teeth, no grin; just a sword and a freaky glowing eye (don't forget that cofagrigus doesn't has its arms out all the time, it can look like a regular sarcophagus, too).
-Second: "the sword it's too plain", and I'm paraphrasing here. Design is all about the contrasts: dark/light, big/small, simple/complicated, and pokemon gets that as very few ever got *digimon!*. eh? who said that? not me, of course. whew, that was close.
Anyway, look at the scabbard, a really elaborate scabbard, lots of lines, a hidden symbol, the tip divides into three, and has perforations on the top.
Then look at the sword, it's really simple; a regular, double edged blade, a very simple handguard and hilt, a cloth that it's a cloth and a hand (not one or the other. It can be easily read as both at the same time) and the only thing out of place is the eye, which looks like a gemstone. Then you put them together, and the perforations in the scabbard make sense now, using the necessary lines in the blade to show an angry face, while the "real" eye gets lost in the background.
As I said, I don't like it, but I recognize good design when I see it. Besides, the worst 6 gen pokemon so far it's litleo. That thing, manticore or not, looks just dumb.