First, I'm so sorry that you're going through this. What an awful situation. Lesley B is right. Because you are married, your husband is the presumed father and will have all of the legal rights and responsibilities of a father even if he's not there and even if you don't list him on the birth certificate. And you can't even get divorced while pregnant. When you do get divorced, whether or not you have any children will come up. You can't lie about that, and custody, child support and visitation will be settled as part of the divorce process.
You can name your child whatever you want. You may be able to lie and omit his name from the birth certificate at birth, but that doesn't negate his paternity or his rights (and responsibilities). At some point, your child will have to be dealt with in the divorce process and it would probably look bad if you lied about this. If his name weren't on the birth certificate, it could later be added. In order to have him NOT be the legal father, another man would have to step up and claim to be the father and be able to prove it via DNA, or later you could have someone else adopt him. The goal of these laws is to ensure that whenever possible, a child has both biological parents, or a bio parent and adoptive step-parent, taking legal and financial responsibility of a child.
As others have mentioned, if you need any kind of government assistance, they will go after your husband first. Even if you don't need help, it will cost you nothing to get a child support order in place. As Gamma G mentioned, if he's the kind of loser who won't pay anyway, then it can be a great incentive for him to stay away. My oldest son's father was a little crazy and unstable when I knew him so the fact that there is a warrant out for his arrest for years of unpaid child support has kept him quiet and far away for many years.
Have you ever heard the phrase "don't borrow trouble"? Try to think of that when you start to project out all of the awful "what ifs" you can think of happening in the future. It's good to prepare and think ahead, but really try to live in the present for now and focus on your pregnancy and your baby. I wouldn't worry about the woman he left you for. If she really is dangerous, document what you know about her in case you ever need to present a reason for him to not have unsupervised visits, but my guess is that that relationship won't last long anyway so there's no use getting all worked up over it. The time line for custody and visitation in divorce is many months just to get to the initial hearing.
You will need to file for divorce. If you have the means to do so, I would find an attorney now who can start on that. You can't complete this while you're pregnant, but you can at least start the process now.
Best of luck to you and your baby - I hope you have a fast and healthy delivery. Staring down single motherhood is scary and sucks, but you can do it - my 5 years of single motherhood were the most rewarding of my life. I wish you the best!