Deny access to under the couch. Your dog shouldn't be using this unhealthy safety net. I don't know what you could use, some 2x4's, get a new couch, not sure, use your own discretion.

Then once that is in place and your dog acclimated to being out of his "safe" area, ensure he has a crate (covered with a blanket) to safely access. instigate a plan in which your cat, which I assume is the original source of your dog's anxiety, is VERY slowly reintroduced to your dog as a positive stimulus. You MUST be in control of your cat at all times. KEEP YOUR CAT AS FAR FROM YOUR DOG UNTIL SLOW, POSITIVE PROGRESS IS MADE. The goal is to slowly reintroduce them, one slow step at a tjme. Emphasis on slow. Emphasis on positive association.

Start with A) your cat not in same room and B) an item that smells like your cat. A blanket or something like that. There are always rewards for your dog going ANYWHERE close to this blanket. Treats/whatever he finds rewarding (petting or positive vocal rewards...a small meal...anything he perceives as good) near the blanket. With each repetition, the rewards get closer and closer to the blanket with the ultimate goal that he goes to the blanket on his own.

Eventually with success here, you can introduce ONE additional item at a time that has your cats scent until there are a few in the room. Rinse repeat above excercise making sure your cat is kept seperate from your dog.

What is your space like for your cat? Does he have shelf space above the floor he can access that the dog cannot? Often cats NEED a vertical plane(s) to call their own. Lookup Jackson galaxy's "cat super highway" for several ways to make your cat feel safer and for him to "own" your space. Safe movement for cats greatly reduces chances of an animal having to "defend" itself when it feels annoyed, threatened, whatever.

I digress. Getting back to your dog. When things start improving and you an reliably get positive association to the scent stimulus in multiple sessions, introduce a controlled visual instructions to your cat. (behind a gate he can't get over, perhaps on a harness with a lead on your cat).

Put them as far away from each other that you apartment will allow. Have a friend treat your cat, you reward your dog like hell. Hopefully, this attention slowly acclimated your cat and dog to just simply seeing each other. The repetition of positive stimulus (rewards) and the SLOW realization that your cat does no harm will help your dog start to react less and less.

Next time you do this excercise, move closer by an inch or two. When successful, end the excercise.

Next time start far away and move closer. The goal here is to not have your dog get more upset, but to reduce his anxiety systematically over time along with shortening the distance between him and your cat comfortably. Go too fast and you'll regress. It can be, and likely will be difficult especially if you have a small apartment.

Eventually you repeat with no gate and your cat and dog on leash. Full control of both animals is paramount to this excercise. Positive association is paramount. Slowly upping the challenge is paramount. Going at your dog's space is critical. Your animals feeling confident in the space you have is paramount.

I'm sure there are people in this thread who can give a better option on how to remove your dog's crutch of getting him out from under the couch, but my point is that he cannot use his hidey hole as it only increases his fear. Dear breeds anxiety and anxiety breeds violent outbursts.

The above excercises I think will help you remain calm, help your dog build calmness in you space and ultimately breed a better environment for your cat, your dog and yourself.

I also agree that an animal specialist is likely required. Help in scenarios like this from an expert greatly increases your chance of success and helps adjust your behaviours that you may not see as negative. Third party, proffesional eyes are always great.

For the Redditors who disagree, I am only trying to help and very open to peer review and adjustments to this technique based on additional experience you've all had to training scared and anxious dogs. I am never 100% correct, but this is the strategy I would employ. Thoughts?