Spike is a dragon, therefor he is also a reptile, reptiles breed similarly to mammals, the male inserts his organ into the female and releases his sperm where it then fertilizes an egg. the egg grows and is then layed to hatch at a later date.
spike is a male so he has a penis and sperm, he can therefor impregnate a pony but only if the pony's body doesn't reject the fetish of mixed species which has happened when crossbreeding certain mammals
So if Rarity and Spike have a baby....
in MLP:FiM Canon Discussion
Spike is a dragon, therefor he is also a reptile, reptiles breed similarly to mammals, the male inserts his organ into the female and releases his sperm where it then fertilizes an egg. the egg grows and is then layed to hatch at a later date.
spike is a male so he has a penis and sperm, he can therefor impregnate a pony but only if the pony's body doesn't reject the fetish of mixed species which has happened when crossbreeding certain mammals