Mary-Kate and Ashley: Winner's Circle

Summary
Simple riding game (the first one for Game Boy outside of JapanI could find), featuring Mary-Kate and Ashley, aka the Olsen Twins, who at the time were apparently popular enough to warrant their own game series.

Game Design
Isometric. The horses look a bit weird but have quite detailed and varied markings; overall not bad, considering the limitations.

Gameplay
There are "Tournaments", where you have to clear three different courses as fast as possible and are ranked accordingly and "Challenges", where you have to collect a certain amount of items in a set time (3 minutes). You can choose from different horses with different stats. Completing challenges gives you access to more horses. You can play as either Mary-Kate or Ashley, not sure if there are any differences (isn't the whole point that they look the same?) and pick colours for your outfit. Completing the three tournaments and all challenges unlocks

Riding Mechanics
You can choose from two different ways to steer the horse in the Control menu: Either "Mary-Kate"(default) where pressing a direction on the D-Pad makes the horse turn that way from the players perspective or "Ashley" where pressing right (or left) has the horse turn clockwise (or anti-clockwise) - I hope this is understandable. I highly recommend changing to the latter mode ("Ashley"), it is far smoother (or maybe I'm just more used to these controls). This is separate from which rider you choose. A is to jump, B to speed up, there's no slowing down and the horse will only come to a halt when crashing into something. The arrow in the lower right corner shows the direction of the next jump/item/finish. The number at the top shows the remaining jumps/items, the time is shown in the lower left corner. If you tear down a fence ("Fault") there are no penalties but "Perfect" jumps will improve your horses stats (only for that tournament). If you get stuck in a jump, it usually helps to press A to make the horse rear and then turn it around so you can try again.

Cheat Code/Password
From GameFAQs: To unlock everything, select "Load Game" in the main menu and enter as password: CMDBGM FCCGKS (make sure to choose your controls/audio beforehand).