The secret that Rockuff and the Starters share is….
They have an interesting Breeding mechanic where the adopt the father’s primary typing. But each the Starters plus Rockruff only have four possible Secondary typings. (Rowlet has Five, thats only because it starts out as a Grass/Flying)
Rockruff’s potential Final typings - Rock/Grass, Rock/Fire, Rock/Water, Rock/Dragon and Pure Rock.
Popplio’s potential Final Typings - Water/Fairy, Water/Psychic, Water/Steel, Water/Ice and Pure Water..
Litten’s potential Final Typings - Fire/Dark, Fire/Electric, Fire/Poison, Fire/Normal and Pure Fire
Rowlet’s potential Final Typings - Grass/Ghost Grass/Rock, Grass/Fighting, Grass/Ground, and Grass/Flying.
They all have varying looks and designs but you are fully aware its the same Pokemon. It is said that the Pure Typings are the true forms of these Pokemon, Rowlet the only non-pure typed starter pokemon its final Evolution is Grass/Flying is said to be the true form of this Pokemon.
All these Alternate forms share the same dex number much like Wormadam has different typings but occupy the same dex number.
The first and second evolutions are standard no matter how you breed them, but the final evolution will showcase the the unique dual typing.