It is a widely held belief that cows and horses cannot walk down stairs. These animals can't easily see the ground right at their feet and the design of their hips and knees makes it hard to shift their weight backwards to stop them tumbling headlong down the steps.
There shouldn't be anything very surprising about this. Stairs aren't found in nature; they are a human invention with proportions that have been designed to suit human legs. The slope of most staircases is over 35 degrees or 4 in 10, which would be a very steep hill indeed. You wouldn't be surprised if a cow balked at the prospect of a 35 degree bank and carving it into small, human-sized steps doesn't really help all that much.
On a shallower gradient, there is nothing magical about the nature of stairs that presents a particular challenge to cows, horses, snakes or any other animal. There are YouTube clips of cows very reluctantly tackling short flights of stairs, so it can be done.