I believe that he either has fin rot, or one of the larger fish is nipping away at his fins. Since he is the only one affected, then it may just be bigger fish picking on him. If it is fin rot, then the fin rot needs to be treated quickly, as it looks like it is already in an aggressive stage. Because of it's unevenness, I would think that it is bacterial related, rather than fungal. First and foremost, quarantine the guppy in a hospital tank. Any sort of large container around 1 gallon will be fine. Make sure the water in the hospital tank is the same temperature at the tank he is already in. Get yourself some aquarium salt and salt the quarantine tank water according to directions -so for a one gallon tank, you will need to mix in 1/2 rounded teaspoon. Let it completely dissolve in the water BEFORE putting the guppy in, so it does not burn him. You should put a small air stone into the quarantine tank as well, so he is able to breathe better within the salted water. After 24 hours, repeat this process, by doing a 100% water change every day in the one gallon tank. It is important to do a 100% change because you do not want to over salt the water by using any already salted water. It is easier to pre dissolve salt in another 1 gallon container, and remove your guppy for a minute while you quickly dump the old water and pour in the new water. Do this for about 5 or 6 days. If there is no visible improvement, you may then want to try medications, which to be honest, I often find do not work at all, but some people get good results from then, so you can try them. Something like API Fin & Body Cure is for treating bacterial fin rot and is available in stores and online at places like Walmart, Amazon, Chewy, etc. Leave the guppy in the quarantine tank and follow the directions of the medication, which will include daily dosing and some partial water changes. Be sure to keep the small tank temperature regulated and aerated if possible. If his fins heal and you return him to the community tank, be sure to closely monitor the behaviors of the other fish to make sure they are not the ones causing his tiny tail to disappear.
ALSO, in case it is bacterial, you are going to want to do at least a 50% water change in your main tank as well. Give the gravel or whatever substrate you're using a good vacuuming to remove any debris and waste that bacteria may be feeding on. Keep an eye out for all of the other fish fins to make sure nobody else has an infection as well.