This footage was taken on July 17, 2010 in Sinaloa, Mexico.
(**July 2015 Update**, looks like this user has made his video private, in which case, here is a different video then from the report below.)
The Onza is a species of wild cat considered to be a legend and one of the numerous creatures studied by cryptozoology. It can also refer to the jaguarundi (P. yaguarondi). It is known as a type of cryptid.
In 1938 and again in 1986, animals in Sinaloa were shot and identified as onzas (the legendary cat that resembles a cougar, not the jaguarundi). Only one viable specimen has been taken: A rancher named Andres Murillo in January 1986 saw what he thought was a jaguar attacking him while deer hunting, so he shot it. It turned out not to be a jaguar, and he brought it to Vega, who owned a nearby ranch. He told Murillo that the specimen he had greatly resembled the onza his father had shot in the 1970s, the skull of which he still had. The specimen was a female that weighed 60 lb (27 kg). The body, not including the tail, was 45 inches (1.1 m) long, and the tail added another 23 inches (58 cm). The cat had the appearance of a cougar with a very long, thin body and long, thin dog-like legs. It had eaten recently because deer had been found in its stomach. These animals were much like cougars but had lighter frames, longer legs, and longer ears and were spotted.
There is another legend that is not well known by cryptozoologists. According to that legend, there are two species of jaguarundi living in Mexico. One of them is usually called onza and the other one is referred by any other local name.
source : http://en.wikipedia.org/wiki/Onza