According to the description of the Ameghino brothers, the iemisch (a.k.a. Iemisch Listai) is larger than the puma (Puma concolor), but with a wider body, shorter legs, and a long, flat, prehensile tail; and is covered in short, stiff, bay (reddish brown or brown)-coloured hair. Its head is said to be short, with large fangs and no or rudimentary external ears, while its paws are described as plantigrade, with three webbed digits on the forepaws and four on the hindpaws, each one armed with a formidable claw. Nocturnal and amphibious, it was allegedly greatly feared, and was said to be powerful enough to seize horses with its claws and drag them into the water. It supposedly lives in caves and other protected areas along the shorelines of lakes and rivers.
From the Florentino Ameghino’s original letter:
The animal is of nocturnal habits, and it's said to be so strong that it can seize horses with its claws and drag them to the bottom of the water. According to the description I have been given, it has a short head, big canine teeth, and no external ears: its feet are short and plantigrade, with three toes on the forefeet and four on the hind, three toaes are formed by a membrane for swimming, and are also armed with formidable claws. Its tail is long, flat and prehensile.
The semi-aquatic canine is called Ahuizotl.