Two animals come to mind: one were saltwater crocodiles in New Guinea and the Solomons. I'm not familiar with any specific American encounters with them (like an attack), but there is a famous, albeit highly exaggerated account involving the deaths of several hundred Japanese soldiers on Ramree off the coast of Burma. Its unlikely the numbers often seen (some say 1000) are accurate, but certainly some did meet their fate at the jaws of crocs in the mangroves.

The other critter they definitely had to face was mosquitoes. According to the US Army "tabulations done after the war, Army medical personnel treated 47,663 cases of malaria in the SPA [Southwest Pacific Area] in 1942. The rate of infection was calculated at almost 251 cases per 1,000 troops in the entire area." Throughout the Pacific War, the Army and Navy struggled to combat both mosquito-infested areas, eventually relying on DDT, and also trying to prevent soldiers and Marines from getting it. Quinine was the primary anti-malarial medication, but the major location for the primary ingredient was owned by the Netherlands, and after 1940, the Nazis, so the US did not have a steady supply for the several hundred thousand people in the Pacific area. The next best thing was Atabrine, but many refused to use it since major side effects were nausea, diarrhea, and yellowing of the skin. It also had to be taken in its proper dosage at regular intervals, which was not always possible in the Pacific theater, especially in combat conditions.