Fine horse leather is known as Cordovan. Cordovan is vegetable tanned over a six-month period.  Cordovan is a by-product, usually from older horses (not slaughtered for this purpose).  Cordovan is the subcutaneous layer that covers the horses rump. Each horse provides two shells, which is usually enough for one pair of shoes.  "The most non-porous leather known, shell cordovan is distinguished by its lustrous waxy finish, superior durability, and suppleness that readily conforms to the shape of the wearer’s foot."

Shell cordovan is a very rare leather.   Most cordovan is sourced from European work horses.  There is a severe shortage of this material at this time, driving prices up.

Horween of Chicago invented this method of tanning on this leather over a century ago and is the only producer of shell cordovan in the USA.  There are several foreign processors of the leather, a couple of which are in Asia but Horween continues to produce, in my opinion, the best shell cordovan out there.

The main colors of cordovan are burgundy (#8) and black.  However, there are some rather difficult colors to obtain such as whiskey (requires a perfect, flawless hide), cigar, ravello, etc.

Cordovan was much more prevalent in the early 1900's, as it may be found in vintage shoes such as Florsheim, Hanover, Nettleton, Bostonian, Alden, etc.  Current brands using shell cordovan consist of Alden, Allen Edmonds, Carmina (Spain), Crockett & Jones (UK), Dinkelacker (Germany), Vass (Budapest), etc. 

While I don't claim to be an expert on this material, I have come across quite a bit of it and have done a lot of research on it.  I would be glad to help distinguish whether a certain item is shell cordovan or calf if I can.