As you may know, Howrse recently made a couple of changes to the Greyfells. One was that you now get 2 diamonds instead of 1 diamond every time you bring your Greyfell back to life. The other change was that you can now give Greyfells' Philosophers' Stones to make them immortal.
So the question came up in the forums (from me actually!) asking if all 12 Greyfells needed to be mortal (have no Philosophers' Stone) in order to produce a Palomino Sleipnir on the 1st of the month. I was asking because I didn't want to put a Stone on one of my Greyfells and then find out that they must all be mortal (have no Stone) on in order to get the Sleipnir.
I was contacted by Jacinta92 who very kindly, sent a ticket to Contact about this. Howrse came back saying that it did not matter - you can give some of your Greyfell's a Stone and you will still be able to get your Palomino Sleipnir on the 1st of the month.
So for those of you who are lucky enough to have all twelve Greyfells', you can relax and look forward to getting that wonderful, new eight-legged Divine.