He is remembered today for the claim that he discovered a previously unknown primate, De Loys's ape, during a 1920 oil survey expedition in Venezuela.

The identity of the animal he photographed has long been established with considerable confidence to be a spider monkey, and the identification as a new species is generally regarded as a hoax.

Between 1917 and 1920, de Loys and his men were searching for oil around the River Tarra and Río Catatumbo at the Venezuela–Colombia border in South America[2] (Bernard Heuvelmans, 1959).

One day, while de Loys and his crew were resting near the Tarra River deep in the jungle, two monkeys suddenly stepped out of the woods, screaming and shaking branches.

When the monkeys threw their own excrement at the terrified de Loys and his exhausted companions, they grabbed their guns and fired at the more aggressive-looking male, but killed the female.

When de Loys finally returned home with the only remaining evidence, a picture which he had placed into his travel-notebook, he basically forgot about his encounter with the unknown monkeys.

When Montandon ran out of convincing arguments in order to support his hypothesis, he tried to bring up some anecdotes based on stories of Indian tribes like about the guayazi, the di-di, and the vasitri or "big-devil" that was believed to attack women.