BlackJack meets Stygius in chapter 44. she first sees him in Luna royal guard armour whilst washing in a pool. He was very flirty with her and got slugged in the face for it. He joins Blackjack on her journey and is a very chivalrous character. it's also interesting to point out that him and his species speak only in high pitch squeaks and squeals, like real bats. but he has a chalk board which he can write onto talk to people.
Its hinted that the mares are in season before she is with P21 and Glory, so I'm guessing it's his. Also... after she had slept with Stygius, she went to hightower and Snips cursed her causing her physical body to die. Any foal that was conceived with Stygius should also have died when the body died. I will therefore, do a huge facehoof if it ends up being Stygius. I will also facehoof if she is pregnant for 9 months... horses and ponies are pregnant for 11 months.