Well, if we're being technical, this galaxy actually takes its nickname from another cetacean. Once a spiral galaxy much like our own, NGC 2936 was twisted all out of shape by its behemoth neighbor. The result? One of the more adorable galaxies in the Water Snake constellation.
This galaxy is another example of our old friend pareidolia in action. I realize this goes without saying, but this isn't actually a giant cosmic porpoise, but it's interesting to see where the resemblance is closest. While the far left of the galaxy does rather resemble a back fin, there's no dorsal fin to speak of, and you have to be feeling pretty generous to count that little blue trail on the bottom right as a flipper. The real key to the illusion is that bright, thick blue arc on top, which really does seem to trace out the cetacean's shape. And, of course, the shape would probably be unrecognizable to us if it didn't happen to resemble a dolphin or porpoise diving into the water — we probably wouldn't remark upon a galaxy that vaguely looked like a dolphin at rest, for instance.
But leaving aside the psychology of its shape, how precisely did this galaxy end up with such an unusual structure? NASA explains:
Just a few hundred million years ago, NGC 2936, the upper of the two large galaxies shown, was likely a normal spiral galaxy — spinning, creating stars — and minding its own business. But then it got too close to the massive elliptical galaxy NGC 2937 below and took a dive. Dubbed the Porpoise Galaxy for its iconic shape, NGC 2936 is not only being deflected but also being distorted by the close gravitational interaction. A burst of young blue stars forms the nose of the porpoise toward the left of the upper galaxy, while the center of the spiral appears as an eye. Alternatively, the galaxy pair, together known as Arp 142, look to some like a penguin protecting an egg. Either way, intricate dark dust lanes and bright blue star streams trail the troubled galaxy to the lower right. In a billion years or so the two galaxies will likely merge into one larger galaxy.