It's been my experience in my daycare that almost all children go through a stage where they are mean. It is something to be concerned about. But I don't think it's something you need to get outside help from yet. If you work on changing this behavior and he still is doing these things in a few months then I would definitely get him some help.
As for what's normal.. This can start as early as 18 months and or not start until later like your dealing with now. I could tell you stories all day long about the types of things kids do. It's just crazy.
As for what to do about it.. I am big on being very direct with kids. I HATE it when I see parents talking to their children in a babyish voice and sugar coating the things they say. In no uncertain terms I tell kids they are mean when they are being mean. I tell them that no one wants to be around someone that's mean and then I put them in a room by themselves. This is not a time out as in a few minutes. This is a time away to play and be by themselves. If they are young enough I'll use a crib. But at your sons age I'd keep him in any room the dogs and or your daughter are not in. You could put him at the table with colors, playdough and other table activities and tell him he is NOT allowed to leave the table because he is dangerous around your daughter and the dogs. Make it clear that you won't have him hurting them and that if he keeps doing this he's going to be playing by himself for as long as it takes for him to start being nice again.
In this day and age a lot of people have grabbed onto new theories that it's somehow wrong and will hurt kids to tell them when they are bad. I swear if people don't wake up we are going to have more and more narcasistic people than we know what to do with. It's not about them it's about their behavior. As parents and caregivers we need to be liberal with our praise when they are good and we need to be liberal with our scolding when they are bad. If a kid is hearing how wonderful they are when they are good, they aren't going to be scarred for life when someone tells them they did a bad thing. I tell kids all the time that no one wants to play with or be around a bully.
I really hate what I see in the world today. The so called professionals out there are scaring parents so bad and confusing them so deeply that no one seems to know how to parent anymore.
You sound like a good mom that really cares about your son. Just remember that kids aren't born knowing right from wrong. And it's fairly apparent by just watching the news or spending some time in any courtroom accross America that kids aren't learning right from wrong like they should.
Sorry for the diatribe! :) I'm going through a stage in my daycare now where some of these kids are incredibly mean to each other and I'm on high alert/guard duty from about 7:30am to 8pm every day. I hate it when I see them being mean to each other and I hate it even more when some parent white washes it by calling them "onry". Hrumph Onry is just a word parents like to use when they are covering up BAD.
Suzi