The sculpture I'm working on is craking all over. So I definitly feel your pain. I changed how I normaly work and of course it means unexpected results. I'm transitioning myself from polymer clay to epoxy clay and so I'm going through the frustration a new medium entails. I'm using the epoxy clay to create a core and covering it with polymer clay. Air is escaping through holes in the epoxy core and expanding the polymer clay causing cracking. I should have known better,I don't know what I was thinking.Anyway, what I do about cracks is to either:
1] Since filling with more PC and rebaking usually doesn't work, I fill cracks with an air drying or chemical curing paste. Although I'm now using epoxy clay, in the past I used Squadron white putty to fill cracks: it comes in a tube, drys very fast and becomes rock hard. You can find it at hobby shops. Or try using air dry paper clay; although this may contract as it drys(I've never used any of these products).
2] I build some detail over the crack. For instance, in your case, you could sculpt a horse blanket over the crack. Make it thick enough to resist cracking.
Hope that helps, as I'm off to deal with my own cracks(oh, that doesn't sound right at all). 
Vincent Ko Schaefer
My WebSite:www.vkschaefer.com
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