Dog roughly punished cat after she ran into him with Toy

My car and dog have known each other for 5+ years since the dog was one, cat has always been mean to the dog, hissed, never attacked him but not nice to him. Dog always wanted to be cats friend but she wanted nothing to do with him. She ran into him with her toy, and possibly scratched him?

He held her down with his mouth solidly for a second, but didn't seem to hurt her and no blood drawn, but she and myself were absolutely terrified

I'm guessing once is enough to look at a behaviouralist?