This one is kind of complicated.
Basically, Bridle Runway (unicorn stallion), Streak wing (Pegasus Mare), and Cave In (Earth Pony Stallion) were fused together through a chance meeting and an accident involving a magical artifact. They kept Cave In's coat, Bridle's hair, and Streak's race and sex.
Through tryingg to rectify their situation, Cave In was all but lost. He was regressed. lost all of his memory, his personality changed some, and his mental capacity was cut down. He's now got the mind of an autistic child, mostly non-verbal. says maybe one word every now and then if he speaks at all. He doesn't front very often except to be a kid. also now he insisnts he is not Cave In anymore. He's Kevin.

The Cutie Marks are: Left, Bridle Runway, a runway model, right, Streak Wing, an athlete and sportspony, and bottom is Cave In/Kevin, a spelunker who found the artifact that caused all this.

Technically, Cave In/Kevin is the oldest and Streak Wing is the youngest.