Your missing banner is the Crawltipede. I can see five Solar Pillar banners grouped together, but there are seven Solar Pillar enemies. According to the Wiki, Drakanian doesn't have a banner and I can match the other five, so your missing banner is the Crawltipede.
The Crawltipede banner was obtainable in early versions of 1.3, but the Worm bug effectively makes it unobtainable. The Worm bug is that only kills to the head of the worm count as a kill and since the only vulnerable part of the Crawltipede is the tail, its kill count is never incremented. If you have an early version of Terraria (e.g. 1.3.0.1), you can get the Crawltipede banner in that and bring it across to your new world.
Unless of course you placed the Crawltipede banner somewhere else and I missed it?
Congratulations on the banner collection! I'm about 75% complete on mine.
Edit:
You probably already knew this, but I'll mention it just in case. The Ghost is an underground hardmode enemy that spawns only during Halloween. You can make it Halloween by changing your computer's clock. It's not particularly rare like the other two are; you can probably get one in the process of doing your normal underground stuff.