To me, this looks like it can be repaired with super glue. Leave a little of the plastic on the outside free of glue so that when the to pieces are joined, the glue won't seep to the surface.If it still doesn't look right, afterwards you can cut any extra plastic sticking out with an exacto-knife.
If you're planning on painting over the pony, some sculpty or similar material can be put over the area (a very thin layer and after you join the pieces together) and then sanded to be the same smoothness of the pony .
I've tired both of these things, and you can't even notice!