Stained glass:

You can assemble the three pieces to make a complete Stained Glass Horse, or you can buy them already assembled. These are up for purchase in the Game Store.

Stained Glass Horses will add +3 to one of the foal's traits when you use it on a breeding pair. The different colors represent different traits, so when deciding which glass to use, consider the parents' strengths and weaknesses and choose accordingly.

Blue - movement

Green - intelligence

Red - heart

Purple - scope

Orange - speed

Yellow - strength

Sven:

Sven will add +2 to ALL traits of the foal when used on a breeding pair. These are pricey (1 million store credits/EBs), but you're paying for a guaranteed boost across the board on your foal's traits.

Keep in mind that Stained Glass/Sven does not guarantee a good foal, but if luck is on your side and you have a good pair, it can definitely help!