Pressie's English name comes from Presque Isle River where one of the better sightings occurred. It is known as to the area’s native people:
...the animal [Mishipishu] is depicted in pictographs at various shoreline sites, either as a spiky cat-like creature (its Ojibway name means "great lynx") or as a serpent (sometimes called by other names).
Modern sightings indicate a serpentine species up to at least 75 feet in length with a horse-like head on a longish neck and a bilobate (whale-type) tail. They swim in a vertically undulating fashion and are said to be dark green to black in color.
More recently, one Randy L. Braun came forward with his tale of an encounter with Pressie that he claims to have had in 1977, and revealed a photograph he took of the beast. That photograph appears in the Youtube clip above. The thing is hideous...or a rock.
h/t Cryptomundo, of course.