Here are some missing specimens and photographs of cryptids. I don't suppose there's much chance of finding the specimens, but perhaps some users on the wiki have seen one of the photographs (not photo #3 though).
The hand of a giant orangutan acquired in Borneo by Henry Keppel in 1846 - "This hand far exceeds in length, breadth, and power, the hand of any man in the ship; and though smoked and shrunk, the circumference of the fingers is half as big again as an ordinary human finger" - alongside a giant orangutan skull also acquired by Keppel. The hand and skull were sent to an unknown museum. [Chad Arment, Cryptozoology: Science & Speculation (2004)]
A giant orangutan, 8' tall when stretched out, killed on Sumatra before 1849. The entire specimen was sent to the museum of the Asiatic Society of Bengal, which was later absorbed into the Indian Society's collection at Calcutta. The current whereabouts of the enormous specimen are unknown; Chad Arment suggests it may have been sent to England. [Chad Arment, Cryptozoology: Science & Speculation (2004)]
A brown hyena shot in Kenya, far beyond its known range, by Richard Meinertzhagen in 1905. Disappeared after being sent to London's Natural History Museum. Possible connection to the Nandi bear. [Hutton, Angus "Legendary Nandi Bear Unveiled," Old Africa #25]
A "huge lynx-like creature," believed to be a Nandi bear, shot in 1936 in the Mau Forest by Jesse R. Coope. The skin and skull were sent to London's Natural History Museum, only to disappear. ["Settler Shoots Beast of Legend," The Hong Kong Telegraph (10 December 1936)]
A pair of "giant forest hyenas," possibly Nandi bears, shot by Douglas Hutton on the Chemomi Tea Estate in Kenya's Nandi Hills in either 1957 or 1958. Their bones were sent to Coryndon Museum at Nairobi, where they were identified as "giant forest hyenas," and have not been heard of since. [George Eberhart, Mysterious Creatures (2004)]
A very large brown hyena with some unusual characteristics, shot in Kenya by Angus Hutton near Chemomi, believed to be a Nandi bear. The skeleton, skin, tracks, and photographs of the animal were sent to Coryndon Museum (where they were examined by Louis S. Leakey and Jane Goodall), then on to the Natural History Museum by sea. All the material disappeared en route. [Hutton, Angus "Legendary Nandi Bear Unveiled," Old Africa #25]
An unusual big cat, officially identified as a "mutant jaguar," but alleged to have been a Smilodon, supposedly shot in Paraguay in 1975 and examined by Juan Acavar. [Karl Shuker, In Search of Prehistoric Survivors, 1995] The fate of the body is unknown; according to some, it may have been incinerated, or covertly sold. The primary source for the story is also unknown.
The Tombstone Epitaph thunderbird photograph, allegedly published in the Arizona newspaper Tombstone Epitaph in 1886. [Karl Shuker, "ShukerNature: SEEKING THE MISSING THUNDERBIRD PHOTOGRAPH - ONE OF CRYPTOZOOLOGY'S MOST TANTALISING UNSOLVED CASES"]
A second photograph of De Loys' ape, allegedly showing the animal beside two men for scale. Supposedly published in a book of mysteries which also mentioned the gazeka. [Karl Shuker, Alien Zoo (2010)]
Photographs of the unusual brown hyena shot near Chemomi by Angus Hutton. Sent to Coryndon Museum, then on to the Natural History Museum by sea, where they apparently disappeared alongside the rest of the material. However, Cunningham-van Someren later saw some photographs of the dead animal, of which he said "I have looked at the photographs but really cannot come to a definite decision though they are hyaena-like". [Hutton, Angus "Legendary Nandi Bear Unveiled," Old Africa #25]
A photograph of a dead Smilodon-like cat lying beside a person. Supposedly seen online. Possible connection to the "mutant jaguar" allegedly shot and examined by Juan Acavar in Paraguay in 1975. [Karl Shuker, pers. comm.]