Gergoth: It Came from 65 Million B.C.
Gergoth, like Galamoth, is dinosaur. He just happens to be an undead one. Zombie dinos are not that common, but that's mostly because dinos aren't that common. When dinosaurs roamed, there weren't many people that practiced the "dark arts." Of course, this is because there were no people, and the dinosaurs weren't really able to cast black magic. So, there weren't many zombies... or any, for that matter.
Of course, there have been the occasional dino corpses. Very, very rarely, a dino or two gets it's self stuck up in the icy tundra, where it dies. The ice preserves it, and then a glacier forms around it. Voila. Even after the ice from the Ice Age starts to receed, the frozen dino has gotten so buried under ice, it's not coming back out easily.
So, the glaciers break up slowly, and the dino, trapped in ice, starts to float around. Over the millions of years, it makes its way from the tundra, up to the arctic, and then, slowly, towards Romania... well, okay, the northern climbs above Romania. ventually, someone finds it, goes "hey, I could make some money off this weird corpse," and decided to try and sell it.
And that is how the corpse is eventually bought and taken to Castlevania. One of the mad magicians there (probably Shaft or the Necromancer) gets the bright idea to bring it back to life, or unlife. They cast thier magics, light their incense, and soon enough, they have a really big zombie.
Of course, why they decided to give it a laser for breath no one is quite sure. Probably because a giant monster, while scary, isn't the most effective thing in combat. You do eventually need it to do more than shamble around and bite things. Besides, lasers are frickin cool.