Essentially to me this one seems unlikely given the circumstances it's based on. As they determined her birthday simply on the release date of (Rescue at Midnight Castle) which was then re-made into (Firefly's Adventure) Firefly being the pony in which RD is based on. This apparently holds significance, as the special was renamed after firefly, meaning the first release should celebrate Rainbow's Birthday. Personally i'm not 100% about this, but based on the rumors about the Mane Six's ages this does match up with Pinkie Pie, making RD the second youngest. Her personality does seem fitting under the Zodiac sign of Aries. But i digress, as none of this is actually canon and just seems like speculation.