Strange Male Cat Behavior?

Question
Hoping you can help. I have a 7 year old neutered orange tabby male cat.  For the last year he's been exhibiting really odd behavior towards two of my other cats.  He will literally pounce on two of my female cats (one 15 spayed female, and the other is 10 and spayed).  He tends to favor one more than the other.  But when I hear the hissing and cat screams, I rush over and his tail is all poofed out and he runs away.  I have noticed that catnip magnifies this behavior to where he will scratch one cat.  So I stopped  giving him cat nip completely.  I've noticed it for about a year.  I tried giving him some calming treats which help some, but don't completely stop the behavior.  I've taken him to the vet and he's healthy. I've been told this is a behavior issue and to re-introduce him slowly. I've done that but it has not helped.  Recently the behavior is even more odd. One night last month I hear rumbling from the basement. I had assumed two cats were possibly fighting but I didn't hear any hissing or anything. But when I got down there he was the only one down there running around with his tail poofed out. He didn't look scared at all.  I did some research on what the tail poofing out means.  They usually do that in defense mode or aggression mode.  Not sure why he would pick only those two female cats to be aggressive to? And I can't for the life of me see how he could feel the need to be defensive. They do not go after him and usually anytime he's within sight of them they start growling. I thought maybe the tail poofing was in response to the other cats screams and hisses.  But then two days ago he comes from the basement. All the other cats are upstairs on various couches or chairs relaxing. His tail is poofed out, but not the hairs on his back.  Then he stars rubbing on one of the chairs and seems calm and content. Not either aggressive or defensive.  At first I thought that maybe his behavior was in response to my one other male cat being diagnosed with HT early in the year. I also thought maybe he could be trying to come the dominant male, but the behavior keeps getting stranger. Thought that it was stress of some kind because they are close buddies, but then why pounce on two specific female cats?  I've tried playing with him more, thinking that it had to do with not getting enough attention, but giving him more play time and attention has not had any affect on his behavior. I thought maybe it was an animal outside (because I live in a rural area) that might be spooking him,  but being winter, deer are pretty much the only animals around mostly.  And it seems his behavior is getting stranger.  I'm at a loss as to what is causing this strange behavior. Hoping you may have some insight as to what may be causing this.

Answer
Melissa,

You could have his thyroid levels checked. The fact that the vet gave him a clean bill of health may only mean that the checkup was fairly basic.  I am not a vet, but I have heard of people giving cat seratonen affecting drugs like prozac.

As long as there is not indication he is aggressive towards you, I would not worry too, too much.  If he begins to show aggressive behavior towards you, then whatever is going on has progressed to a stage where he may become an aggressive psycho-kitty with you. Although this is unusual, it can happen.

I am concerned that the behaviors are getting stranger and more pronounced over time. Is it possible there is, indeed, something in the basement upsetting him?  Is it possible there is a strange outdoors cat he is seeing through a window that is agitating him?  If he begins this behavior, can you put him in an interior room with no windows for a "time out" for an hour or two?

As I said, I am not a vet, but I would guess that there is something progressive going on. You may have to do some more detective work to find the cause if you can. It would sure be nice if they could tell us what they are experiencing.

Please let me know what you find.

Best regards... Norm.