The Lingering Pokemon, according to the Pokedex anyways...
For a more serious answer, it's not really known exactly what species ancient Dreepy was. The closest we can get in game is its Pokedex entries, specifically the Sword and Violet ones:
After being reborn as a ghost Pokémon, Dreepy wanders the areas it used to inhabit back when it was alive in prehistoric seas.
-Sword
It has a habit of biting at Clauncher even though it doesn’t feed on them. This is said to be vestigial behavior from when Dreepy was alive.
-Violet
The dex entries mention Dreepy living in prehistoric seas and it bites Clauncher as a vestigial behavior from when it was alive, but this still doesn't give us much insight on its species, except that it was an aquatic creature.
Bulbapedia states:
Dreepy seems to be a combination of an undead prehistoric amphibian, the Diplocaulus, and a dart or missile. The Dragon type may be a reference to the Olm, a type of salamander native to central and southeastern Europe that was believed to be a dragon offspring by locals.
It's likely that a prehistoric Dreepy would more closely resemble one of if not both of these creatures, as well as being not dead. But unless we get more info on Dreepy's origin by the Pokémon Company or in the games, Dreepy's prehistoric characteristics fall unto speculation.
Or maybe we'll get a Past Paradox form of Dreepy that is literally just a wooden dart who knows?
Hope this helped.
