Constant speed

[Tugdual]

I recently came accross this simple question which I found difficult to answer.
Consider the very common function y=sin(x). When you draw it with a computer, you see that the plot is accelerating or decelerating. Nothing surprising since the derivate of sin is cos.
Now suppose you want to do an animation showing an ant walking on your sinus at constant speed. What would then be the equation to draw this?
Can you find a general answer applicable for any equation?