A dogs Pant is to cool themselves down when they overheat, as they can't sweat like we do. Also to oxygenate their blood. A normal Pant on a warm day is never to worry about many people automatically believe a dog Pant means needing water.
If you feel this panting is for another reason then a dog can Pant because he or she is in some sort of discomfort or in pain. My dog some years ago was panting out of the ordinary. I found he had an abcess on his anal glands. Must have been uncomfortable for him. He was fine after antibiotics. You can see if your dog has any sours between paws, in the mouth as your dog could have ulcers or bad teeth?
You do say he is 15 years so I'm sorry to say your dog is an old dog now. beagle life expectancy is 12-15 years.
The best thing to do would be to see a vet or at least speak to one over the phone.
Very possible your dog is having respiratory or heart problems. There really is not much you can do at home and he really needs to be seen by a Vet. Some take Care Credit or do you have a friend or family member that might help? So sorry, I hope you can get some help.
As much as you don't want to hear this, your dog MUST be seen by a vet. Panting can indicate anything from pain to difficulty breathing. If he is having difficulty holding his urine it could also mean there is kidney failure involved.
Fifteen years is very elderly for a dog, so this is no time to mess around with not going to the vet. Talk to your vet and explain the difficulties you having with money, but explain that you are worried about your dog and want to do right by him. Maybe your vet will agree to at least see him and see if he thinks that it's something minor or whether it might be kinder to euthanize him rather than allow him to live in discomfort if you cannot afford treatments. It could also be something that, whether you have the money or not, at fifteen years of age it would be kinder to let him go than to let him be uncomfortable with something that cannot be fixed at his age. I know that's not what you want to hear, but I am trying to be honest with you.
Please come back and let us know what the vet says.
Ghilly