What To Do
If you think your horse is lame, then stop riding them and seek a veterinary lameness exam. Perform the Whole Horse Exam, putting emphasis on determining which limb is lame, characterizing when and how the lameness appears. Look at and feel the affected limb for swelling, heat, digital pulse. Always pick up the hoof and carefully evaluate the sole. Assess for lameness at walk and trot (if appropriate) to the best of your ability, then get your vet involved.
When in doubt, rest a lame horse in a small enclosure (no larger than 20' (6 meters) diameter until your vet can determine the nature of the problem, and provide treatment options. It is especially important to confine a horse that is on an anti-inflammatory medication like bute. Confined, they are less likely to experience less pain, over-exert and worsen the injury.
What Not To Do
Do not give your horse pain relieving medications and ask them to perform their job, without a diagnosis. You may cause them to over-exert, worsening the injury.
Do not listen to the majority of non-veterinarians, many of whom will be happy to give you strong opinions about what is wrong with your horse and even tell you how to treat it. In most cases, this is nothing more than a guess. There is a tiny minority of non-vet professionals who have real lameness experience and knowledge just from their vast equine experience. They will be the first ones to encourage you to seek a diagnosis from a knowledgeable veterinarian.