Milotic: you only get this thing by maxing out it's beauty stat, this makes it perfect for a narcissist.
Kabuto: obligatory fossil joke.
Chandelure: the immortal needs to have at least one ghost pokemon to chill by his side forever. this one's also nice and elegant.
Delcatty: it's pretty. Dorian's vain.
Relicanth: this Pokemon's existed for like a bajillion years without changing, hello there Dorian, meet your immortal pokemon friend.
Snorlax: I won't lie, Dorian would see Snorlax and basically think "this thing is an absolute waste of space, I want it so badly." he will end up regretting this later.