Hi,
The Messages App (iMessages is just one service it can join) can actually join 5 services.
The app started life as iChat
The Original version had the Ability to have AIM and Rendezvous (Later this became Bonjour) accounts.
At the same time Apple valid aim did a deal with AIM that meant that the @mac.com names they were issuing at the time were also valid AIM Screen Names.
iChat 3 saw Jabber added.
iChat 4 saw that split into Google and jabber (Google run a Jabber server but it does not follow Jabber naming conventions)
iChat 6 saw Yahoo added.
Messages in Mountain Lion (OS X 10.8) saw the start of Messages which was then called version 7
Mavericks and Yosemite both had their version numbered as 8
Version 9 comes with El Capitan.
You can add a existing AIM name in the Messages App in Preferences > Accounts and using the + icon.
If you have a valid AIM name in System Preferences > Internet Accounts it can get added automatically at an Upgrade.
The fact it appears in the Main Window suggests the account is Enabled in the Preferences > Accounts.
My list is so long it is scrollable

The names that show are the ones in the "description" field in Preferences > Accounts.
It used to be you could drag them in to the order you wanted but this is no longer possible.

7:58 pm Saturday; April 16, 2016
iMac 2.5Ghz i5 2011 (El Capitan)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad