Drinking and urinary problems in horses are not as common as some people think - horses have robust kidneys, however they can produce very slimy colourful urine without there being a problem.
To see if your horse is drinking or urinating excessively - monitor intake and output over a period of at least 24 hours, a good indicator is the state of the bedding, if it is soaked through when it never used to be, it is worth investigating.
One of the most common causes of increased drinking in horses and ponies over thirteen years old is Cushings Diseases. Most psychological drinking and urination problems are associated with underlying stresses such as a bully in the field, an ill fitting saddle, or even a rider horse mismatch. Back pain, mouth pain and stomach ulcers can also cause some horses to drink excessively to take their minds off their discomfort.
Remember some horses just get satisfaction from urinating on nice clean bedding.
If you are overly concerned then it is always worth asking your vet to check your horse out.