@Itsthinking  
You’ve predicted the future.

@PonyPon  
Still doesn’t make sense.
 
Aside from the fact that he’s never displayed signs of being a runt, animals wait until the egg hatches to determine that.

@QueenCold  
So revealing one of his parents, but not really?
 
Could work.

@Trickquestion  
Smallest of the clutch?

@Itsthinking  
I’d be surprised if we ever found out, so I made my own little headcanon.
 
Cimmerian doesn’t hoard trinkets. He hoards memories of faces contorted with fear or agony. What better way is there to collect such treasures than by attacking a nest? He kills Spike’s mother and destroys most of the eggs, not noticing Spike. Spike’s father fights Cimmerian but loses. I haven’t decided Spike’s father’s faith yet. He either dies by Cimmerians hand or manages to survive and live the rest of his life in shame.  
If he does survive, there might be a chance he and Spike meet again. This is how I personally imagined that would go:
 
Spike finds a rugged dragon with a thick Scottish accent beating the crap out of a hydra, and is so impressed that he follows him around, much to the large dragon’s dismay. The dragon acts bitter and unpleasant in general. Over the course of the episode the dragon warms up to Spike, but remains largely surly. Deep down he is more likable than he lets on and eventually even Spike notices. Despite the fact every nice moment gets relentlessly shot down, the two do bond and gain a mutual respect for each other. It is never stated that the dragon is Spike’s long lost father, but so many hints are dropped that the audience can put two and two together.  
Eventually, with the help of the dragon, Spike finds his way back to Ponyville. The large dragon silently promises to keep an eye on the young one.

@Itsthinking  
How would they tell? He was in an egg.

@QueenCold  
Well, damn. Maybe his parents didn’t want him. Maybe he was a runt.

Cimmerian should be a villain for the show.

@QueenCold  
I thought it was more “no idea who or where his parents are” than them being dead.

@QueenCold  
It’s really ambiguous as far as I know. We would hope he’s an orphan, as bad as that sounds, because the implications of the other possibilities are not pleasant.

@QueenCold  
Spike’s origins are unknown.

@Itsthinking  
That makes the assumption that dragons live with their parents for long. From his interactions with other dragons, he’s already old enough to live on his own.

@Itsthinking  
Didn’t they say he’s an orphan?

This ain’t gonna end well for the green guy.
 
@Itsthinking  
Call me crazy, but I imagine them being proud of a dragon defending “his territory” by keeping watch over it. Ya know, if they really had to sugarcoat it.

Shame they can’t show Spike’s parents without him going back to live with them.