First-time novelist Mark Jacobson, 42, created a Frankenstein for himself when he cooked up Gojiro the mutant lizard. Weighing in at 50 tons and towering 500 feet, the reptile ''just sort of took over,'' says Jacobson, who spent eight years writing the much-touted Gojiro (Atlantic Monthly Press). What he had by the tail was the tale of a monstrously good-hearted lizard and a Hiroshima orphan who stumble onto a mission to rescue the planet. While writing it, Jacobson borrowed money from friends, brought home one, two, three new babies to his New York apartment, and gnashed his teeth. ''People were afraid to ask me what I was doing because I'd just growl at them,'' he says. Jacobson knew how to write. One of his first magazine articles was turned into the TV series Taxi by James L. Brooks. In 1979, inspired by Godzilla films, he invented Gojiro, the movie. That didn't pan out, but the big lizard is finally screen-bound: Brooks, who now produces The Simpsons, just made a second deal with Jacobson.