If you're wondering what this is, let me break it down for you. No, this is not a researched theory. Two, I am not a Giga-Ultra-Omega-Super Fan of this Pokemon. Three, this is my own personal theory. If you have any other theory, feel free to tell me in the comments section.
Sigilyph is the Avianoid Pokemon, standing at 4.7 ft. tall. The mystery behind the design is most likely based off of something from Egypt, or some other countries with ruins. Now then, count the number of limbs (The 4-bottom limbs, and the 8 top area limbs). The total number is 16. Now then, if you count the 6 Black limbs, you would get a total of 22 limbs. Let's talk eyes, shall we? The Avianoid's eyes are blue, like normal human beings, however there is only "one" eye, and that would be the one coming from the thin "head." However, there are said to be 2 more eyes on this vast Pokemon's body, and those would be on the torso. Last thing I'd like to point out is another theory I created. The "Avianoid & Unown" theory. If you look at the thin head of the Pokemon, it resembles the Unown letter "I."
This Pokemon has a lot of different moves to it's disposal. Here's where things get really interesting...This Pokemon can learn the move hypnosis, and can learn Dream Eater as well, to restore its own health. Now then, I guess this isn't involved with the "Moveset" Category, but I found this really interesting. According to the Pokemon White Pokedex, the entry is this: "The Guardians of an Ancient City, they use their psychic power to attack enemies that invade their territory." What I personally believe, is that Sigilyph uses Hypnosis to make the enemy fall asleep, then eats the enemy's dream. Now then, A wild Sigilyph COULD end a human's life, that is unless the human woke up in time.
I personally believe that the Sigilyph are hiding something much scarier...Would you believe me if I told you that the Sigilyph are actually Unidentifed flying object-like creatures? Before you start raising your "Horseshit" signs, let me explain. They always fly on the same route, but never another route except that specific road to the ruins. Here, we turn to our valuable friend: Desert Resort in Pokemon Black1/White1. Notice how the Sigilyph only appear in the Desert Resort? The Same route where there are some ancient ruins? Now then, prepare to have your mind possibly fucked with. Remember how I said it was the "Avianoid" Pokemon? The idea came from the Nazca Lines found at multiple locations of the past, which are also ancient. Some people love to mess with us in the real world, and scream something about Unidentified Flying Objects. It appears that there have been some "UFO Sightings" in these locations, which could have inspired the idea for Sigilyph itself.
From Alien Pokemon, to Real World UFO Sightings, Sigilyph still remains a mystery for the ages of Pokemon. Believe what you want, but don't scream it across the road every 5 seconds. That was my theory that I came up with.