Manaphy and Phione
There are a pair of legendary Pokémon that are like this: Manaphy, when paired with a Ditto, produces a Phione egg. They do not evolve.
Illumise and Volbeat: Honorable Mention 1
Illumise and Ditto can produce Volbeat eggs, and vice versa. They do not evolve.
Nidoran Lines: Honorable Mention 2
Nidoran♂ paired with a Ditto can make a Nidoran♀ egg), for example. See the full chart to see what combinations work.
Some evolutions require a certain gender of Pokémon, but will produce offspring of both genders. A Vespiquen, for example, can produce eggs that contain male Combee, which do not evolve.
Same dex number, different forms
Due to the way Alolan and Galar forms work, it is possible for a parent to produce offspring that cannot evolve into the same form it is.
- Breeding regular Ponyta will produce Galarian Ponyta eggs in Galar, for example. Same for Pokémon like regular Sandshrew in Alola.
- Pokémon like Raichu (which don't have a regional pre-evolution) are an interesting case, as the Pikachu offspring of a regular Raichu can only evolve into Alolan Raichu if in Alola, while an Alolan Raichu will produce regular Pikachus that will evolve into regular Raichus when not in Alola (so far, this means Galar).
I don’t remember any of this coming up outside of the main games, and the honorable mentions only work as of gen 5.
In all other cases, the bred egg will be the same species, or a pre-evolution of one of the parents. Both parents don’t need to be the same species, however.