My Illogical Theory
I believe that Volcarona is the signature Pokemon of Alder.
I believe this is for a few reasons - firstly, Alder's (supposed) signature type is Bug, in which he had three of in Black and White. In the manga (I couldn't find anything on anime) he had Bouffalant, Accelgor, Escavalier and Vanilluxe, which is a predominantly Bug-type team.
Secondly, an Elite Four Pokemon trainer wouldn't have Normal as their signature type. That's sort of logic really; Normal is a pretty unstrategic type that the Champion wouldn't be using as their signature Pokemon. They'd use something a lot more powerful, like Dragon.
And finally, the games tend to overpower the anime a little bit. No offense to the anime lovers out there, but it is the case... :P
Hope I helped. :)