I only saw the first two (three? I can't quite remember) episodes of Merlin, so my initial reaction to this post is, "Wait, what's this about a unicorn? Wait, what?"

My second reaction is, "Pffsht, Artur clearly has a Cleffa."

A psychic Meowth would be among the Delta species; these are card-game only Pokemon who have alternate types. Off the top of my head, these include a psychic-type Treeko, an electric-type Psyduck and Golduck, and a fire-type Ludicolo. There are others, obviously, but I don't feel like dragging my cards out to check.

The Delta species seem to be confined to a specific region, although again, I can't remember and don't feel like checking. I think that they're the result of some kind of radiation in the area (presumable delta waves), and it is spreading farther away over time, affecting more Pokemon as it goes along.

It is entirely possible that whatever force pushed the Pokemon from their world into Merlin's world would have the same power to turn them into Delta Pokemon. Possibly it even had Delta waves involved, but only one in a million was affected.

Clearly I'm giving this too much thought. What would Gaius have?