Iemisch
The Iemisch (a.k.a. Iemisch Listai) is a supposed monster from Patagonia, specifically in the mountains near the Lake Colhué region. First attested to by Florentino Ameghino in 1897, a full study on the creature was published in the 1955 book On the Track of Unknown Animals.

From the original letter:

During follow-up research by Bernard Heuvelmans, the local population described the iemisch as a mixture of a jaguar and otter, though by some accounts it was as big as an ox. He claimed the creature was also referred to as a tigre d'acqua, similar to the ahuizotl. It reportedly could move as quickly on land as in the water, and was described as having a "soul-wrenching scream".

Robert Lehmann-Nitsche, a German anthropologist working in Argentina, claimed to have a sample of the iemisch's skin given to him by a local rancher. He stated that there were small bone plates embedded in its skin, which protected the creature from arrowheads. The rancher reported that the sample was found nearby human remains, leading him to believe that the iemisch had been hunted.

Robert Lehmann-Nitsche and Santiago Roth would eventually publish more iemisch tales in their work The Mysterious Mammal of Patagonia, Grypotherium Domesticum. Their conclusion was that the iemisch must be an unknown type of otter.

Upon peer review, contemporaries of Nitsche and Roth noted that though they had spent equal amounts of time with Patagonian natives, they had never heard of such a creature. Later scholars also cast doubt on research by Heuvelmans, noting that the word 'Iemisch' isn't associated with any language spoken in Patagonia.

See also 
 Ahuizotl

References

Cryptids
Purported mammals