You do have two choices:
A: Do not give the new pop account the description 'POP Account', by just replacing 'POP Account' with a name you want the new e-Mail account to be identified as.
B: Rename the existing 'POP Account' to another name -
01: In 'Mail' select the 'Mail, Preferences...' menu item.
02: Click on displayed window's 'Accounts' toolbar icon.
03: In the left side list of 'Accounts', locate and click on the account identified as 'POP Account'.
04: In the right side list of text edit fields locate the 'Description:' text edit field and change it from 'POP Account' to anything else - such as what you have enter in the 'Full Name:' text edit field (a couple of lines below 'Description'), which is typical of many.
05: Click on another account, in the left side list, or the window's close (red jellybean) button to force the 'Save' sheet to appear. Only after clicking the 'Save' button will the left side 'POP Account' change to the new 'Description:' entry.