Patryn of Elvenshae said:
It's a manufactured weapon. As such, it cannot be used as a primary or secondary natural attack. Treat it just like you would a human with two longswords (or other TWF combo) using the TWF rules, including however you determine when the penalties apply.
I think you misunderstood what I was asking. I know the tail blade is a manufactured weapon. I was wondering how it interacts with a creatures natural attack routine (i.e. claw, claw, bite, tail blade) not, for instance, if a lizardman was wielding two swords and a tail blade together (in which case the multifighting rules apply). Two weapon fighting rules don't apply if a creature is only wielding a single manufactured weapon.
Say you have a creature with a +6 BAB and that normally the creature has a bite, claw, claw attack routine with its bite being its primary attack. Normally when wielding a sword in one hand that creature could make two attacks with its sword +6/+1 and then follow up with its other natural attacks assuming those limbs/teeth are free. So then this creature would follow up its two sword attacks with its empty claw and its bite both at +1. In this case, the creatures primary attack is the sword, and not its bite. It's bite is instead counted as a secondary attack which means the attack roll is rolled at +1 instead of its usual +6 and its strength bonus is only counted at half on its damage roll.
The tail blade however is different in the sense that it doesn't replace/occupy one of the creature's claws in a full attack routine. It makes use of a limb that was previously useless in combat. If the tail blade is its primary weapon the creature's attack routine would become +6/+1/+1/+1/+1 (tail blade x2, claws x2, bite). If the creature has Multiattack as a feat all of its secondary attacks would gain an effective +3 to hit making its attack routine +6/+1/+4/+4/+4 and the creature never has to bother with Multiweapon Fighting. Effectively the tail blade grants two extra attacks per round to the creature with little to no trade-off. That's a pretty sweet deal. There's no reason why every creature with a tail wouldn't want to strap one on.
On the other hand

if the line "or it can make
a tail blade attack in addition to its other melee attacks" (emphasis mine) means that the tail blade can only be used to make a single extra attack that would seem to indicate that a high BAB wouldn't matter to a creature wearing one. I wanted to know how people interpreted this weapon. What's the creature's attack routine look like while wearing one?