A rabbit's urine is thicker than most other animals due to the way they metabolize calcium. It can also be different colors depending on the vegetable matter that has been eaten. If it is really concentrated or the rabbit is straining to urinate...it could be a sign of a urinary tract infection and/or sludge/stones. If the rabbit is dehydrated the urine will be very thick. This is one for a rabbit savvy vet to determine to make sure everything is OK. In the meantime, you might want to increase the hydration to see if you get any improvement. One of the easiest ways to do that is a few hand fulls of wet leafy greens such as romaine lettuce...don't overdo it, but a generous portion.