What are the ten most popular pets in the US?

That will depend how you wish to quantify the numbers: by the total of homes or by the total number of animals.

According to the AVMA, who cited the 2007 U.S. Pet Ownership & Demographics Sourcebook, if you go by the total number of owned animals, cats appear to be first:

  1. Cats 81,721,000
  2. Fish 75,898,000
  3. Dogs 72,114,000
  4. Birds 11,199,000
  5. Livestock 10,995,000
  6. Horses 7,295,000
  7. Rabbits 6,171,000
  8. Other Birds 4,966,000
  9. All Others 3,664,000
  10. Turtles 1,991,000
  11. Hamsters 1,239,000
  12. Ferrets 1,060,000
  13. Guinea Pigs 1,004,000

If you go by the number of owner households, dogs are first:

  1. Dogs 43,021,000
  2. Cats 37,460,000
  3. Fish 9,036,000
  4. Birds 4,453,000
  5. Horses 2,087,000
  6. Rabbits 1,870,000
  7. All Others 1,182,000
  8. Turtles 1,106,000
  9. Hamsters 826,000
  10. Livestock 728,000
  11. Lizards 719,000
  12. Guinea Pigs 628,000
  13. Ferrets 505,000