Perfectly Normal Beast

Perfectly Normal Beast are a type of animal from the Hitchhiker's Guide to the Galaxy. which migrate to the planet Lamuella twice a year for six days before disappearing.

Physiology
The following is an extract from Mostly Harmless-

 "A quadruped with rather odd migratory habits that grazes across the Lamuellan land, providing sustenance for its inhabitants and sandwich meat for the Sandwich Maker. A Perfectly Normal Beast is a bit like a cow, or rather a bull. Kind of like a buffalo in fact. Large, charging sort of animal. They come from a point slightly to the east of the Hondo Mountains, where they suddenly appear. Then they sweep in their thousands across the great Anhondo Plains, and, well, vanish really.  "

They migrate in a very large herd of thousands upon thousands of Perfectly Normal Beasts, sweeping in magnificent array across the Anhondo Plain. In the early pale light of the morning, as the great animals charged through the fine steam of the sweat of their bodies mingled with the muddy mist churned up by their pounding hooves, their appearance seemed a little unreal and ghostly anyway, but what was heart-stopping about them was where they came from and where they went to, which appeared to be, simply, nowhere.

 They formed a solid, charging phalanx roughly a hundred yards wide and half a mile long. The phalanx never moved, except that it exhibited a slight gradual drift sideways and backwards for the eight or nine days that it regularly appeared for. But though the phalanx stayed more or less constant, the great beasts of which it was composed charged steadily at upwards of twenty miles an hour, appearing suddenly from thin air at one end of the plain, and disappearing equally abruptly at the other end.