This is a very old bug from the first day of release that still gets on my nerves. To reproduce it is simple, not sure why its still around after 6+ years. But anyway it happens to dragons only, when you log in you get the biped Equipment window if you logged out with your dragon equipment window open.
Steps:
- Make sure you are a dragon
- Press E to open your equipment window
- Exit game by ALT+F4 or press the X if in window mode
- Log back into the game with same dragon character
- Notice you now have a biped equipment window
Easy to reproduce eh?