dog who has never been aggressive before; now being aggressive to border pup

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lap dogs come in- all sizes
Barked: Sun Oct 7, '12 6:43am PST 
Neehi has always been the dog that defused aggressive attacks and fights. If another dog starts a fight with him he turns his head away and stays calm, and the other dog stops the attack. He has also never started a fight in the 2 years I have had him.
We have had a Pitbull staying with us for 5 months now, and will be here for at least another 40 days. Neehi and him are best friends who play and sleep together. But in the last week I have seen Neehi go after Bogey (the pittie) 3 times now. I just do not understand why?!
Bogey sleeps downstairs with the girls who rent our basement. One of the times Neehi went after Bogey, one of the girls was feeding my 2 dogs and Bogey some training treats. The 3 dogs kept getting closer to her, and closer together. I told the girl that she needed to get them to back up and sit because Lola (my food-aggressive-dog-with-other-dogs-dog)and Bogey were getting to close to each other. Sure enough, Lola snapped at Bogey with some vocal outrage, although she didn't touch him. Neehi then pounced on Bogey and was biting him. We split it up, and made all dogs move away from each other. After a few minutes we let them move around and Neehi went straight for Bogey, circled him and jumped on him again.
The other 2 times, I don't get at all. Lola and Neehi were playing and Bogey was wondering around. Neehi stopped his play with Lola, looked at Bogey, circled him and pounced.
Bogey may be a pittie, but is a big babysmile He just cries and tries to get away when this happens.
When Bogey first came I was worried how Lola would treat him, and she bossed him around a lot at first, but never did more the "correcting" fights, never hurting him. Lola rarely corrects him now, except with a snarl, growl or snap, and even then this is not often. But now Neehi is acting like I have never seen him act and I do not like it.
Can anyone tell me what is happening to my boy?
Stella- "Blue"

Puttin' my freak- on!
Barked: Sun Oct 7, '12 10:37am PST 
2 males and a female?

lap dogs come in- all sizes
Barked: Sun Oct 7, '12 10:52am PST 
yes, Neehi and Bogey are males, and Lola is female. Lola as always been the bossy one and Neehi the "whatever" kind of dog. Bogey is super submissive.
Like I said before, Bogey has been here 5 months with no problems, until this past week or two, and now Neehi is.. well.. being a bully to him.
They still play with each other.

Dr. Watson

Not a wiener- dawg!
Barked: Sun Oct 7, '12 10:59am PST 
So Neehi, is a bitch, and she is the aggressor. She has always been sort of bossy, correct?

lap dogs come in- all sizes
Barked: Sun Oct 7, '12 11:21am PST 
lol... nosmile
Neehi is a boy, and has never been an aggressor before now.
Sorry if I am confusing.
Lola is the one and only bitch, both in sex and attitude, but she is not fighting with either boy at all... except for a snarl/growl or snap once in a while.

The Muddy- Princess
Barked: Sun Oct 7, '12 12:19pm PST 
How old are Neehi and Bogie? It sounds to me like Neehi is protecting or guarding your girl. My Duffy (my 6 y.o. male) is quite protective of Selli (my 8 y.o. female), even though Selli doesn't need protection. Duffy's redeeming quality is that he just grumps and doesn't take it any further.

I\\\'ll do- anything for a- treat!
Barked: Sun Oct 7, '12 4:44pm PST 
Many dogs who have never shown aggression will display some sort of "attitude" towards certain dogs somewhere around the age of 2-3 years of age. Puppies generally tend to get along with everybody, but fully mature dogs aren't quite as tolerant, especially towards other dogs of the same sex.
Lupi snarked at her first exuberant puppy when she was 2.5 yrs. old. My friend's male dog started barking at bigger male dogs when he turned 2. My other friends Frenchie has begun to show signs of fear towards bigger dogs since she's turned 3. It's pretty normal and can be easily dealt with using some counter-conditioning (reward training)
It's highly plausible that Neehi sees Bogey as his competitor for all sorts of good things, and may be resource guarding. I would help him associate the Pittie with good things, by treating and praising him when Bogey is near.