Now this just got interesting. Perhaps out of all the villains in Narnia, Tash is the most mysterious.

You should check out "Into The Wardrobe" on Youtube; he gave insight about theories on Tash. It's never clear where he had originated.

One possibility that he was a star (like Coriakin and Ramandu). He rebelled in the skies, and fell, eventually settling in Calormene.

Another theory is that he was originally from Charn, and that Jadis had allowed herself to possessed by Tash. When Jadis fled the North, Tash may have gone South, and eventually, Calormen was established, and they begin to worship him.

It would have explained that both Jadis and Tash claim the rights of their lawful prey (Edmund for Jadis and Rishda for Tash) What else? Tash only has one line throughout the series-

"Thou hast called me into Narnia, Rishda Tarkaan. Here I am. What has thou to say?"

Tash is mentioned in The Horse and His Boy, and is not seen until The Last Battle. It's almost like Smaug in The Hobbit or the raptors in Jurassic Park or Shere Khan in Disney's adaptation of The Jungle Book.

One will never know where Tash originated or how even came to be in Calormen.

There's a famous quote from Robert Oppenheimer, from around 1965, where he is recalling the first Atomic Weapon test during WW2, and he expresses his somewhat sorrowful regret at his involvement by quoting a passage from Hindu scripture;

"We knew the world would not be the same. A few people laughed, a few people cried, most people were silent.

I remembered the line from the Hindu scripture, the Bhagavad Gita.

Vishnu is trying to persuade the Prince that he should do his duty, and to impress him takes on his multi-armed form and says, 'Now I am become Death, the Destroyer of Worlds.' I suppose we all thought that one way or another."

Although Oppenheimer didn't reveal this thought publicly untill after the time Last Battle came out (so unlike The Deplorable Word, I dont think there is any thematic connection to Nuclear Apocalypse) it is still the quote that comes to mind for me...

Tash is the living manifestation of the End of Days. He is the Destroyer of Worlds.

As a child I always assumed that Tash was just a different mystical being in the Narnia universe that the Calormenes were attracted to as something to worship. 

As it stands I have never seen Tash as Allah because I believe that the legends that the Calormene culture is based on are far older.  As an adult I have always assumed he was just a representation of other gods from other religions and faiths who were different in nature to the Christian God and Jesus.  The fact that Lewis portrayed Tash as real being in the LTB suggests to me that he has respect for the faiths of other people in the world and allowed that what they believed could be just as valid as what Christians believe.  He just didn't feel that it was right for him to believe.  I have not seen Tash as a villain so much as a representation of another type of belief that seemed to suit the Calormene.   

I have a couple of favorite hypotheses about Tash. Both have at their root the idea that he is some kind of legitimate spirit-being within the Narnia world's ecosystem, who went Bad and came to be worshipped as a God rivaling Aslan.

One, based on the Satan-parallel, is the possibility that he is some kind of rebellious fallen Star.

The other, which I am actually more fond of these days although it has less Biblical resonance, is that he is a minor god/nature spirit native to Calormen, who got too big for his britches and started denying his creator in his pleasure at being in charge of people and looked up to by them. Specifically, I like the idea of him being natively a Desert-spirit - the deathly smell and the bird of prey motif may be because of desert scavengers who take what dies before it reaches the other side? But as a God of War he can accelerate the process of finding prey, by predating on his followers' fallen enemies?