It's not really rare. Form Delta Species above they created new "delta" pokémon, that have different types than the ones they used to.
Examples:
Lightning-type Dratini
Fire-type Mankey
Steel-type Magikarp
...
and so on...
It has a black circle in the bottom corner. That means it's excessively common. (Despite the fact that I don't think I ever pulled one from a pack, actually. Blah, I only bought like 6 packs of HP, anyways.)