Despite being more or less ageless with an undefined beginning, he was still a kid once. A tall, lanky one à la Fluttershy, but just a kid.
 
The primordial (original) alicorns came into physical being as foals when they settled upon the new worlds, to grow up and discover themselves among the mortals. Asriel, a sweet-natured but odd entity that was physically younger than most, was lucky to have made it this far; his piebald coat is unique to him (in terms of the primordial gods), and his mind is fractured by the horrors of the War.
 
Becoming a Nightmare is an inevitable reality for the fragile god, sooner or later.