1. Pointers

Using pointers is a common way to get multiple variables back.

void add(int* tiger, int* leopard) {
    *tiger++;
    *leopard++;
}

2. Arrays

We can do the same job by passing an array as an argument of a function which is expect to be modified, but I wouldn't recommend this as a general solution because every single element of the array has to be the same data type.

int* add(int arg[]) {
    arg[0]++;
    arg[1]++;

    return arg;
}

3. Struct

Structure is one of beautiful features in C even though there is no way to explicitly involve functions with it. It just works like a default data type such as int.

typedef struct _zoo {
    int tiger;
    int leopard;
} zoo;

zoo add(zoo arg) {
    arg.tiger++;
    arg.leopard++;

    return arg;
}

4. More?

How about this? Would it work?

zoo* add(int tiger, int leopard) {
    zoo v = {tiger+1, leopard+1};

    return &v;
}

The answer is no. It does not work because the stack of the function add is going to be cleaned up when the function is done. That means all the local variable of the function will be gone! Also, you will see a warning message like function returns address of local variable when you try to compile it.