The notion of enumeration type was added to the C language in the ANSI C standard, first published in 1989, after the C language had already been in widespread use for a decade. The variable declaration

enum { kCat, kDog, kCow, kHorse } animal;

declares an integer variable animal, which is supposed to have only four permissible values, identified by the symbolic constant names kCat, kDog, kCow, and kHorse. It's common to see enum used together with typedef, to define an “enum type” (especially in a .h header file) which can subsequently be used to declare any number of variables with the same set of permissible values, e.g.

typedef enum { kCat, kDog, kCow, kHorse } Animal;
...
Animal a1, a2, a3;

This looks very nice, and it's tempting to think that we have succeeded in defining a truly new data-type, but in reality this is all just syntactic sugar. The C compiler treats enum variables just like integers (the number of bits used is compiler-specific). The user-specified symbolic names are assigned actual integer values by the compiler in an ordered sequence, usually starting with 0, and are also treated just like integers. That is, the declarations above are entirely equivalent to

#define kCat    0
#define kDog    1
#define kCow    2
#define kHorse  3
 
int animal;
int a1, a2, a3;

The following code would therefore be quite legitimate, and we quickly see that our hopes of defining a truly new data-type are nothing but empty hopes after all:

int x = kDog;        /* assign a symbolic Animal value to an int                   */
int y = kDog + 99;   /* use a symbolic Animal value in an int-valued expression    */
animal = 100;        /* assign an arbitrary int value to a variable of type Animal */