I believe this would be Krill, even though it is one of the smallest organisms, by weight, is THE most abundant on earth.
Humanity EDIT: This person is incorrect. In terms of species, Humans comprise about 100 million tonnes of the Earth's Biomass. In comparison, the Arctic Krill contributes probably 500 million tonnes. As an animal group, Copepods form the largest animal biomass on earth. Source: Wikipedia
The most populated animal more than humans are ants
If the animal is eating regularly and it is still losing weight, you should probably take it to the veterinarian as it should not be losing weight. Otherwise, it depends on the animal. Most pets, particularly cats and dogs, gain weight when they are fed more cat or dog food.
clearly i think the most abundant mammal on earth is Man*Human*. We dominate almost every where on earth..
Bacteria are the most abundant species on earth. Their total number cannot be exactly known, but estimate puts it at around 5 x 1030. There could be as many as one billion species. A link can be found below to the study and its results.Bacteria are not a species of life. They are a domain, which is the broadest classification of life. The most abundant domain on Earth is certainly bacteria and the most abundant species on Earth is likely a particular species of bacteria.
insects There are a few abundant land animals. Most of the abundant animals are insects.
The most abundant metal in the earth crust is aluminium.
cholesterol
probaly birds
Oxygen
water
Muscle
muscle
Oxygen
Oxygen
By mass Oxygen
The most abundant category of animal species is the insects