This "distinction" is simply an ad hoc categorization designed to avoid evidence once it has become impossible to evade any other way.
It's similar to the "distinction" between "a serious charge if proven true" and "not rising to the level of impeachment" used by Clinton's defenders. Any charge that could still be denied by questioning the evidence could be admitted to the former category; anything that had been nailed on Clinton beyond doubt was assigned to the latter category (and thus didn't really count). We saw the Lewinsky affair shifted from the former to the latter before our very eyes after the DNA-stained blue dress surfaced (just as we saw Eohippus --> Equus move from "macro" to "micro" once the fossil record was filled in too thoroughly to deny any longer).
Proponents of evolution often attempt to discredit creation by pointing to occurrences of microevolution, such as speciation, adaptation, etc. To the evolutionist, microevolution is vindication for their belief in the much larger macroevolution. Their belief is that if these microevolutionary changes have enough time to accumulate, then eventually this will lead to a macroevolutionary change. And therefore, in their way of thinking, if microevolution is a well established fact, macroevolution must logically be an established fact as well.
Pure nonsense!
Micro is allele frequency change below species level and macro is accumulated changes at species level or above (with the understanding that the steps of accumulation can only occur from species to species). You cannot have a jump by Genus or above. That is what creationists would like us to believe macro is, but it isn't. Sorry to disillusion you.
I think you have misperceived the scientific definitions of microevolution and macroevolution. You can't use creationist definitions and make a coherent argument. Macro is accumulation of allele changes sufficient to produce speciation. Speciation is defined by science.
I am no expert on horse linage, but it is clear that horses have speciated over millions of years, your apparent claim to the contrary notwithstanding.