I agree with Shona that the most likely "native ferret" in Argentina
is a grison.  There's a lovely pic of one at
http://www.americazoo.com/goto/index/mammals/277.htm
 
I want one!!

 
Sue, when you were in the Dominican Republic, I wonder if this is the
animal that you spotted.
 
The Tayra that Sukie found is also a good candidate, and after looking at
those pics, I want one of those too!
 
When dealing with common names, and in another language, it's hard to be
sure which animal is being referred to.
 
Shirley
[Posted in FML issue 4753]