One of the best ways to find specific Pokemon in Pokemon Go is by going to nests, locations in the real world where the same species of Pokemon spawns at a very high frequency. But unfortunately, there are a number of Pokemon that simply do not have any nests at all.
This includes all of the baby Pokemon, which can currently only be obtained through eggs and that therefore do not nest.
Then there are the ultra-rare Pokemon like Unown. Unown is currently the rarest Pokemon in the entire game, and its rarity is exacerbated by the fact that there do not exist any Unown nests anywhere. Larvitar, another very rare Pokemon, does not nest either.
In addition, there are some Pokemon like Mareep that you would assume would have nests, but for whatever reason, they actually don’t. Mareep isn’t known for being particularly rare in the previous games, but it is much more difficult to come by in Pokemon Go than players expected.
The fact that a species doesn’t spawn doesn’t mean there are no ways to pinpoint its spawn. Some of these Pokemon do not nest, but they have common spawns. Therefore you may, for example, encounter an area that spawns Mareep more often than some other locations do, but this doesn’t technically qualify as a nest.
Here are all of the second generation Pokemon which do not currently nest in Pokemon Go: