Although the question was asked rather vaguely, there are a few places here where help would probably be needed.
Here are some points, in order :
1. Comment the outer main out
2. missing semicolon
struct Zoo { .. };
3. typo at return
return zoo;
4. wrong direction at cout, missing overload for data type zoo
cout << zoo[i].name;
5. use c++ datatype for container
vector<Zoo> zoo;
6. Dont forget to add animal to zoo container
zoo.push_back(animal_create(number, name, weight));
7. missing parameter at output
output_animal(zoo);
8. doubles the weight of ALL animals
double_weight(zoo);