All the monsters--X.
It doesn't make sense that a word would start with "X"--so, by extension, the "x" monsters are artificial, alien things that make no sense.
Xag-Ya & Xeg-Yi
These Monster Manual 2 beasts are magic living matter/anti-matter balls with tentacles. Not as stupid as they could be, not as full-on freakish as the Xorn.
They're a little abstract for my taste, though the comments for these posts have included so many good monster ideas that I bet one of you out there has thought of a nice spin on the Gygax-anagram monsters so I'm gonna sit back and see what y'all have to say in this case.
The Xorn is so weird and makes so little sense that you have to give it props just for that-- it comes from the elemental plane of earth to here to mug people for their precious minerals. Implying:
A) Things like gold and rubies are hard to find on the Elemental Plane of Earth, and
B) It can't just use its move-through-stone power to collect them itself.
Plus it "moves its molecules" around plus it's called a "xorn" plus it looks like a stone Humpty Dumpty with a nose-arm and a mouth on the top of it's head. Seriously, someone was like "Hey, there's a monster in the Manual for every letter but X--let's take drugs until we think of something!"
Doctor Who box set
59 minutes ago