Complex sentence: He won everything Rowe entered him in, and by sizable margins... all except the Futurity stakes where he came in an unaccountable third, beaten by another star, the filly Artful.
Split into simpler sentences: He won everything Rowe entered him in, by sizable margins. All except the Futurity stakes where he came in an unaccountable third, beaten by another star, the filly Artful.