When using ./summon EntityHorse ~ ~ ~

{Type: 4, Tame: 1, SaddleItem: 1}

Or ./Summon EntityHorse ~ ~ ~

{Type: 3, Tame: 1, SaddleItem: 1}

The dreaded red text reads: "Unable to summon object", despite being able to do so in earlier versions.
If this error was intentional, then no harm done.