Lip tattoos are most commonly used on registered Thoroughbreds (bio-horse) as a form of identification. They consist of a single letter indicating year of birth followed by five numbers. In Throughbreds, the tattoo is applied to the inside of the upper lip. Use the calculator below to figure out what foaling year corresponds to what letter (for Thoroughbreds):
In ponyplay, the tattoo is also applied to the inside of the pony's lips but can be on either the upper or lower lip (or both). Moreover, a ponygirl (or ponyboy) is obviously not limited to the exact encoding scheme imposed by The Jockey Club and words or phrases may be used instead.
Thus on a human pony, a lip tattoo can be of any design and represent whatever you wish. I personally love the idea of a lip tattoo because it is very private, and even those who see you completely naked will not know of its existence unless you tell them. For this reason, I think showing someone else your lip tattoo is surprisingly intimate.
If you identify with Thoroughbreds (as I do), you can use the lip tattoo code of The Jockey Club to make up your own registration number based on your year of birth. And, don't worry if you screw up: you can always get a new one on the inside of your lower lip (just kidding!).
Since I identify with Thoroughbreds, I will briefly go over the format of their lip tattoo (other breeds will have different formats, for example Standardbreds don't use the letters I, O, Q, U or Y in their tattoos). Thoroughbred lip tattoos consist of one letter followed by five numbers (e.g. A34567):
Thus if a horse's registration number is 1234567, and it was foaled in 1971, it would receive the following tattoo on the inside of its upper lip: A34567
The five digits following the letter can be whatever you want. If you are really committed to ponyplay and identify with Thoroughbreds, there's certainly nothing wrong with getting a lip tattoo with the proper letter code followed by the last digits of your registration number with a human pony registry. Personally, I am not quite that committed, and already have another idea for a lip tattoo in any event, which I hope to have done soon.
A quick note: the lip tattoo letter codes for Thoroughbreds do not technically have set letters for any of the years prior to 1945. However, since there are only 26 letters in the alphabet, the first letter code is repeated every 26 years: namely Thoroughbreds born in 1945, 1971, or 1997 would all have a lip tattoo starting with the letter "A". So if you were born before 1945 and are interested in what your letter code would be, you can back calculate, or enter a year in the calculator above and it will tell you the corresponding letter (it works for years post-1944 as well).
There is also a human pony reigstry, where both ponies and trainers can register. Just like a bio-horse breed registry (but open to human ponies of all breeds), they will issue a unique registration number which can be used to identify the pony.