In a nut shell, in creationist terms the differences in dogs are all micro-evolution yet the differences from the horses ancestor to the current horse species is "macro evolution" and not proven.
But by comparing every step of horse evolution and measuring the differences and seeing that they are still smaller then the differences between many dogs (which was micro evolution apparantly) then you can undoubtly conclude that a horse was evolved and that either:
1. the idea that it is macro evolution is wrong and it was in reality micro evolution, but opening that flood door will simply lead to larger, bigger examples up untill the whole fossil record is just micro evolution and just acceptable.
2. concluded that macro evolution is just a whole bunch of micro evolution + time..
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