In "Jungle of Stone", William Carlsen mentions that Frederick Catherwood traveled in Egypt with a seven barrel pistol in 1832 to deter thieves. Was a pistol like this common in 1832? Was it custom made? What would such a pistol actually look like?

There are a few candidates for the gun in question:

A pepperbox pistol like this one by London gunmaker John Twigg circa 1785 or this one this one by British gunsmith Samuel Nock circa 1850 or this example by Allen & Thurber of Worcester, Massachusetts circa 1847. This pistols were proto-revolvers that fired one bullet at a time. Pepperrbox pistols were quite popular between the early 1800s to the mid-1800s, when they were superceded by proper revolvers.
A volley pistol, which had multiple barrels pointed in the same direction. One trigger pull set off all the barrels.
A duck's foot pistol, a variant of the volley pistol, which had multiple barrels pointing out at different angles.

My money's on a percussion cap-using pepperbox pistol similar to the Nock or Allen & Thruber example. 1832 is right around when these kinds of pistols were hitting the popular market -- flintlock versions of pepperbox pistols like the Twigg gun I linked had been around since the late 1800s, but they were far less reliable and not especially common. This would would make him an early adopter of what would become a very popular pistol design.