Every new account created on Felisfire comes with 6 engineered Felidae eggs. The feli created from these eggs are special, and are commonly referred to as the 'Original 6' or O6 of the user. Three are male and three are female (though if the player in question should unlock the Lab Changer feature before using them, it is possible to change their gender before creating the custom feli). The eggs and feli are in every way the same as any other egg or custom that may be obtained, with the exception of them being locked to the account they came with. Locked feli cannot be sold, traded, or given away in any other way; they can, however, be sacrificed.
O6 have a small title stating 'this is an O6 feli' on their profile, below their date of birth.
Though the O6 are locked to the account they came with, due to the rule forbidding them from switching hands in any way, they have to be transferred or sacrificed when a player switches accounts. Because of their locked status, you will have to specially request a staff member to do it in the appropriate forum. It is not allowed to leave any O6 behind on your old account.
Currently, this is the only reason for feli to be locked to an account (though some users might have more than 6, see below.
The account-locking of O6 as well as their profile label were introduced to the site on in the first half of 2011, after continuous trouble with the site rules concerning O6. When FF was first released, there were no rules regarding O6 switching hands between players and they were in every sense the same as other customs. To prevent exploit of the free customs and discourage creating extra accounts (though that in itself has always been against the ToS), it was later forbidden to give away any O6 or to trade them for off-site currency. It eventually became forbidden to trade or sell them at all in any form (as is the current rule). Because account locking of felis was not yet possible, the O6 label was invented to prevent accidents on both the seller and buyer's side. As the label did not work retroactively staff went through customs and started labeling them as O6 when they had reason to suspect them to be. All users with previously existing O6 that were missed by staff were asked to declare them in a thread (which is still open, in case users return from hiatus or the like) so they could be labeled.
These rules are not enforced retroactively. Because many O6 were traded when this was still allowed in the rules, there are some older users that have more of these feli than their own O6. If there is reason to believe they are someone else's O6, they are labeled and locked to the account of the user they are currently on. Sometimes it is impossible to find out if feli that switched hands were originally O6 (for example, their creator has gone inactive before the rule changes). As such there are likely still a few unlabeled and unlocked O6 floating around with low IDs.