It matters because if you declare something that cannot be viable, for now, due to limits of biology and technology that will likely be overcome in time, as non-living, then the entire argument that life “begins” at conception will need alterations. That will open a whole new can of worms.
You cannot assume it will ever be viable. For instance, does a molar pregnancy have a soul? I doubt it, since it is generally just a clump of tissues. So, unless and until your scenario can be shown to work (half human, half horse) it’s a moot point, since it may never ever be a viable being.
Before you were born, the egg that is you was in your mother.
It was formed in her body when she was still in the womb.
That’s how old you really are.
Same thing for your mother herself. The egg that made her was in your grandmother.
So it’s in a way untrue to say “life begins at conception...” or things like that, because the egg that we all “are” was certainly alive and well way, way before our births.
But it doesn't live because it is not viable. If God wants the Horse-Human hybrid to survive, he will allow us to find a way to do it. So far, He doesn't seem to show much interest in interspecies mixing.
Life begins at conception can only refer to a viable life form, by default.