Horizontal flip animation in Keynote
Insert the two images (other object or group to be transitioned).
Select the first side and from the animation inspector add a Build Out effect of Flip. Disable Bounce.


Select the second side and from the animation inspector add a Build In effect of Flip. Disable Bounce.
Choose Build Order and set the second build (the build in) to Start: With Build 1.


Set the second side to be in front of the first side by right-clicking on the second side and choosing Bring to Front. Position the two sides on top of each other.