Are you using a tile based system?

if so, the problem could be represented as such:

For the most natural movement, you would want the enemy (green) to pass into each of the tiles which the line passes through.

The distance for both x and y can be found like so:

That can be simplified down to 3:1, 4 times.

If you think about the movement in steps:

start with the direction which is longer, and move that number of tiles in that direction, one tile per step, then move the distance in the other direction, one step at a time.

The closer to 0 one value in the ratio is, the more straight the path of motion. If the lengths of either x or y are prime, then this will result in a path where there is one movement on and and one on y.