If you were born under Scorpio, he'll give you a 5th Element on your birthday. He can breed with Sagittarius to give birth to Capricorn.

From Scorpio's page:
"Scorpio is one of the horses of the Zodiac.

If you were born under this sign, Scorpio will give you 1 x 5th Element every year on your birthday, if you log in on that day (based on the site's update time).

What's more, the 22nd November, when the daily update takes place, a Scorpio horse is offered to a player native of this sign.

By collecting all 12 horses of the Zodiac, you will get the horse, Serpentarius"

However, he cannot breed. Why the option is there is because they decided to have it. Maybe in the future.. who knows.

My bad, he can breed - indeed with Sagittarius (didn't show up for me anymore, as I already used the cover option - again, my excuses.)