User blog comment:Miskos3/Fairy Tail 511: The Hell of Starvation, Review/@comment-4449950-20161119124218/@comment-30338537-20161120174920

Maybe Larcade was naturally born to Mavis, but Larcade then died due to the Curse as an infant when either/both his parents showed love to him.

Zeref was then heartbroken on losing Mavis and Larcade, but managed to resurrect Larcade same way he did to Natsu. Zeref decided to never again show affection for Larcade so that he wouldn't be killed again and thus Larcade grew up not feeling "fatherly love".

Maybe Zeref used the same "resurrection pod" (the thing a dead person is placed in when they go through the process of revival) when he resurrected Natsu and Larcade, hence their identical smells.

According to Sting there is a difference in smell even among siblings/parents/etc (dunno how Gray and Silver could be immediately connected based on smell, unless Natsu is a better smeller than Sting) so perhaps smell is "decided" the moment one is born or reborn in this case. If the same "pod" was used for Natsu and Larcade, they would have identical smells, but these are still different from the other Etherious created. If the other Etherious were created from other "pods", they would each have a unique smell.

Long story short, Larcade is an Etherious revived in exactly the same fashion as Natsu. Zeref added a little extra in both of them: Natsu tuned to kill Zeref, Larcade tuned to kill Acnologia.

Theory may be a bit Farfetch'd, but not entirely impossible I'd say so.