Well, we already have a pet dragon (as you mentioned) that anyone can get - it simply requires commitment and skill to obtain (namely, you get it by unlocking the Dungeon Master trophy for Argan Jungle).
Adding a random rare mob that drops this (or a very similar) pet, and that can get killed by anyone comparatively easily, would devalue these efforts. No one would really bother about the trophy then - just kill the rare mob and there you go, here's the pet. I wouldn't want that.
There are pets you can get from random mobs - well before level 30. I don't think adding a dragon pet that overshadows them all would be the way to go. In addition, it's another reason for you to reach level 30 - and to team up with other players to get there faster.
