Keep in mind that many if not most of the ponies in this show have names that are lifted directly from or are derivative names from the original 80s show. And back then they were simply marketing ploys just like Pound Puppies (bleh) or Transformers (awesome). The pony universe likely lacks any naming convention as stringent as what even the most basic of societies enforce on earth.
It basically boils down to us thinking too much about it. They have names, the names fit them, that's about it.
On another topic, who's cranking out pony avatars?