What is "Other" account on OS X Yosemite login screen?

This indicates you either enabled the root user or you connected to a directory service (Open Directory or Active Directory) for the ability to login with network accounts.

Directory Services: Open System Preferences > Users & Groups > Login Options

Look for a Network Account Server in the main pane of Login Options and edit accordingly if one is present.

Root User: "Open Users & Groups preferences, click Login Options, then click the lock icon to unlock it. If necessary, type your password, then click Unlock. In the Network Account Server section, click Join or Edit. Click Open Directory Utility. Click the lock icon to unlock it, then enter your administrator name and password. Do one of the following: Choose Edit > Enable Root User, then enter a root user password in the Password and Verify fields.

Choose Edit > Disable Root User.

Choose Edit > Change Root Password, then enter a new root user password."

https://support.apple.com/kb/PH14281?viewlocale=en_US&locale=en_US

I was able to remove the "Other" account from the login screen with this command:

sudo defaults write /Library/Preferences/com.apple.loginwindow SHOWOTHERUSERS_MANAGED -bool FALSE