SciFi horror novel about people who regressed into primitive animals. Two scenes I recall involved a woman jogging, I think on a beach, and she was attacked by “animals”, that turned out to be these regressive forms of humans. The other scene was people in a house, and they had the lights turned out because the mutant regressive human animals were prowling around just outside the windows.
As mentioned in a comment above by Bruskovich I'm fairly certain this would be a Dean Koontz book, more specifically Midnight.
From the wikipedia entry (has a lot of the plot, maybe avoid or enter carefully if you really want to re-read):
First: Janice Capshaw, a nighttime jogger, is pursued and then killed by a pack of mysterious and nightmarish beasts while she is jogging along the beaches of a coastal town named Moonlight Cove.
The second scene is probably at the house of the guy who notified FBI of something going on. He is in a wheelchair I think.
One Rainy Night by Richard Laymon was published in 1991, so a little out of the time frame you indicated, but is a reasonably close match otherwise.
It features a small town in which a mysterious dark and sticky rain falls, and any of the inhabitants that the rain sticks to begin behaving like aggressive animals. It features the stories of many of the residents as they attempt to avoid being killed by those who have changed, including scenes that my memory tells me are at least similar to the ones you describe.
You may be remembering the story as by Stephen King because he wrote a glowing blurb that was prominently featured on the cover of at least some editions.