Re origin of "pumpernickel" involving Napoleon's horse:

Random House and American Heritage give about the same etymology. This is RH:

[1750-60; < G Pumpernickel orig., an opprobrious name for anyone considered disagreeable = pumper (n) to break wind + Nickel hypocoristic form of Nikolaus Nicholas (cf. NICKEL); presumably applied to the bread from its effect on the digestive system]

To clear up the "nickel" component, AH gives "demon" and "rascal" as senses of the German "nickel".

Also note that RH dates "pumpernickel" a bit before Napoleon's birth.