The undead being covered under Know: Religion is a little odd. It makes sense since undead are sort of the responsibility of the cleric, but beyond that you could make a case for it being Know: Arc (undead are generally animated by magic), or Planar (tied to the Negative Energy Plane). I'm not saying it should be that way, only that Know: Religion doesn't leap into people's minds as the way to suss out a vampire's weaknesses. I was making a monster hunter npc one time and I wanted to make sure he had the right skills to know a little about any type of monster, and Undead being Knowledge Religion surprised me a little too.
Girallons are what, Magical Beasts? Makes it a Know: Arcana check to know about them? I could see a lot of monsters like that being under Know: Nature, because even if they aren't "natural" to us, they are natural in the context of a fantasy world. (I can't see a ranger being stumped by a girallon only to have the wizard step up and say, "No problem, I minored in four-armed gorillas back at the Collegium Magisterium.")
Let's say the necromancer is really sneaky. He kills some girallons using poison, so they won't be all cut up and bloody. Then he Gentle Reposes them, animates them as zombies, and then casts another spell to make them faster. (Haste, perhaps, at least until Undead Alacrity makes it in from the 2e Necromancer's Handbook.) The girallon's attack the party.
The party will roll Know: Arcana to get the skinny on girallons, but have no idea why they don't seem to be taking damage from piercing or bludgeoning damage. (For most players, that'd be the dead giveaway right there. So to speak.) At that point, just asking for a Know: Religion check is going to tell the pc's what they're facing, so the check should probably be rolled in secret. (Even if you can trust your players not to metagame, the unknown quantity is part of the fun, otherwise you could just tell them their fighting hasted zombie girallons and be done with the knowledge checks altogether.)
I'm not suggesting this is what happened, I'm just proposing it as a way to illustrate a point. Sometimes just asking for the Knowledge check gives as much information as the check itself.
Girallons are what, Magical Beasts? Makes it a Know: Arcana check to know about them? I could see a lot of monsters like that being under Know: Nature, because even if they aren't "natural" to us, they are natural in the context of a fantasy world. (I can't see a ranger being stumped by a girallon only to have the wizard step up and say, "No problem, I minored in four-armed gorillas back at the Collegium Magisterium.")
Let's say the necromancer is really sneaky. He kills some girallons using poison, so they won't be all cut up and bloody. Then he Gentle Reposes them, animates them as zombies, and then casts another spell to make them faster. (Haste, perhaps, at least until Undead Alacrity makes it in from the 2e Necromancer's Handbook.) The girallon's attack the party.
The party will roll Know: Arcana to get the skinny on girallons, but have no idea why they don't seem to be taking damage from piercing or bludgeoning damage. (For most players, that'd be the dead giveaway right there. So to speak.) At that point, just asking for a Know: Religion check is going to tell the pc's what they're facing, so the check should probably be rolled in secret. (Even if you can trust your players not to metagame, the unknown quantity is part of the fun, otherwise you could just tell them their fighting hasted zombie girallons and be done with the knowledge checks altogether.)
I'm not suggesting this is what happened, I'm just proposing it as a way to illustrate a point. Sometimes just asking for the Knowledge check gives as much information as the check itself.