Social media snaps appear to show the large farm animal crammed into the boot of a light blue 04 reg Volkswagen Passat on Friday evening.
South Wales Police said officers were looking into whether animal welfare or road traffic offences had occurred.
In some images it looks as if the young cow is looking out the window of the estate car.
The car was spotted going apst junction 42 of the eastbound carriageway. It is not immediately clear why the animal was in the car.
The RSPCA said it was "extremely concerned" about the images.
An RSPCA spokeswoman said: "This is a completely unacceptable way to transport a large, farm animal.
"This represents an animal welfare concern but also a risk to the safety of the driver and other road users.
"We would strongly urge people never to transport an animal like this."
Overloading a car can see motorists fined up to £300 and given three points on their license.
The AA said overloading vehicles can affect the handling and braking and put too much strain on tyres.
Source: Read Full Article