Why did Ridley really name it the "Deacon"?

Good point that, I wonder.

Well, Bishop was taken :D

Cardinal and Pope don't sound right.

Deacon roll perfectly off the tongue as a creature name.

Indeed they originally refereed to it was Bishop but this was changed to a Deacon because well we have a Bishop in the Franchise, and i dont think they really looked into it deeply what a Deacon actually was as far as the Church.

This creature is in no way connected to any Church Figure, it just got that name because of the shape of the head etc.... and the name is just a working title name like Neomorph is, the Deacon is never referenced as such in the movie and as far as the Engineers go we dont know what name they referred to it as.

Ridley did not name it the Deacon. The art department nicknamed it that because Ridley thought the head looked like a Deacon's pointy hat.

It was called a proto-xenomorph early on, and Ultramorph in Spaith's script. Damon Lindelof changed it to Deacon in his later script rewrites, but that was because the design already had the nickname by that point.

Ridley Scott dubbed this creature the Deacon, based on the pointed shape of its head, similar to a Bishop's head-dress.  (Prometheus Blu-Ray gallery notes) 

"Ridley said " It looks like a bishop's mitre, the evil deacon's pointed hat"" recalls Arthur Max. -- (Prometheus: The art of the film. p186)

The term: The Evil Deacon has come through the ages. Who exactly was the original Evil Deacon? One can be traced at least back to the 12th Century,  Hugh of Horsea who Alfred Duggan informs us that the renegade clerk became famous in Kent as the Evil Deacon and we can find from various sources that he was known for having gut his foot on the neck of the Thomas Becket, and scattered the rest his brains and blood once the top of the head had been chopped off.

"To the vociferations of Hugh of Horsea , a military sub-deacon, "where is the traitor?" -- (A History of England: Combining the Various Histories by Rapin, Henry, Hume, p180)

"What was the Deacon in Prometheus all about?"

Do you mean some meaning other than that's what the proto xeno organism looks like when hosted by that particular faction of Engineers?

As far as the color, I suppose that's what the art department and Ridley thought looked cool and different. Why was the xenomorph black, or the neomorph white. Something from the host DNA dictated it I suppose.

Kethol exactly what you said. Nothing significant about the name, just a pointy head.

It was named after Joey Deacon, a famous disabled guy in the 1980s who threw one of his shoes off a ferry one time.