While some silly laws really existed or still do, articles such as these rarely cite where a particular law may actually be found in the present Massachusetts General Laws, or in any previous laws going back to colonial times. This is partly due to journalistic laziness, or simply the desire to be humorous without actually believing that there is or ever was such a law. Many articles may be found online that repeat the Massachusetts "gorilla in the back seat" law, yet none cites the chapter and section of the law, because there is none. The gorilla law is, unfortunately, an urban myth. We are not quite sure when or how it started.
A diverting article from Boston Magazine in 2002, "My Short Happy Life in Crime", about one man's experience with some of Massachusetts' silly laws, mentions his failed attempt to locate the gorilla law.
It is possible that the misunderstanding began when someone read the first part (only) of Massachusetts General Law ch. 90 s.22H, which reads: "No person shall transport an animal in the back of a motor vehicle in a space intended for a load on the vehicle on a public way unless such space is enclosed or has side and tail racks to a height of at least 46 inches extending vertically from the floor, the animal is cross tethered to the vehicle, the animal is protected by a secured container or cage or the animal is otherwise protected in a manner which will prevent the animal from being thrown or from falling or jumping from the vehicle."
Ignoring all mention of a requirement for an enclosure, or the portion of the vehicle (presumably a truck) intended for a load, one could then simply extrapolate from the word "animal" and substitute the word "gorilla", to make for a hilarious sounding, but supposedly true law.
The current law may derive, in part, from the Revised Laws of Massachusetts (published in 1902), ch. 52, s. 16: "Whoever leads or drives a bear or other dangerous wild animal or causes it to travel upon or be conveyed over a public way unless properly secured in some covered vehicle or cage shall be punished by a fine of not less than five or more than twenty dollars."
This was from a time, of course, when most vehicles were not motorized; and unlike the present law, it only covers “dangerous wild animals”, not pets such as dogs. (We might assume that a gorilla is a dangerous wild animal.)
Both Massachusetts and federal laws regulate the importation, transportation, and possession of endangered species, and the gorilla is listed by the federal government as an endangered species. You may see the complete list of endangered species at 50 CFR (Code of Federal Regulations) 17.11. Massachusetts does not list the gorilla specifically,as it is not native to Massachusetts, but M.G.L. c.131 s.23 lists wild animals that may not be owned without a permit, and 321 CMR 9.02(c), on Exotic wildlife, states that: "Any vertebrate taxa not listed in 321 CMR 9.02 shall be presumed to be wild, and shall be subject to provisions of M.G.L. c.131."
In other words, since the gorilla is both a wild and endangered species, you may not have one as a pet. (Zoos are exempt from restrictions against owning endangered species.)
But what if the gorilla is not owned by you? If you happen to find yourself with a free gorilla, can you drive with it in your car?
No, even then, transporting a gorilla would be subject to federal law 9 CFR 3.87 on the transportation of nonhuman primates (NHP), requiring a separate, specially designed enclosure for the nonhuman primate. (You would also need a permit.)
Therefore, your choices are limited.
If you happen to find yourself in a car with a gorilla, your best bet to avoid a fine would be to exit the vehicle.