Possible Clues:

• Comet, e.g.
• At one buck, this makes the last of 18 down expensive, by the sound of it
• Caribou used by Santa Claus
• 'Dancer or Prancer, say (8)'
• Red-nosed Rudolph is one
• Animals with peculiar prehensile tails, certainly not quite right
• Rise up in river displacing a ruminant
• Check on what sounds like costly animal
• Animal about to get in river by edge of moor
• Domesticated animals are heading off in river close to border