The Lions of Tsavo were maneless, perhaps due to environmental variables, although maneless lions are not unique to Tsavo. Their taste for man may have been due to an outbreak of Rinderpest at the time, which may have depleted their normal prey. The legend surrounding this event is almost entirely based on the books written by Patterson which became run-away best sellers for their day, and made Patterson a good bit of money. It is possible, if not probable, the count of 140 deaths may have been trumped up a bit. Patterson certainly set himself up as the hero of the story, which certainly fit in with the Western notion of the "great white hunter" of the period. It is known that he killed both lions (both nearly nine feet long), and that they did indeed kill and eat humans. It is also possible that they did this because they may not have been able to kill and eat their normal prey as the jaws of the two show some sign of unusual dental disease. They now reside in the permanent collection of the Field Museum of Chicago, but the government of Kenya is moving to try to obtain the pair.
There is only one scene involving an animatronic lion. All the other shots were used using two real life lions named Bongo and Caeser. The same lions also appeared in the film George of the Jungle (1997).
The rifle used by Val Kilmer is a "Lee Speed" sporter, most likely in .303 caliber. The movie rifle is historically correct and most likely came from a South African movie prop supply house. BSA made these rifles from 1892 until at least the 1930s in .303, 7mm, and 8mm. BSA offered several different versions and options on these rifles. The term "Lee Speed" was used for commercial rifles. These rifles were mostly manufactured by the BSA Company, who also made Gov't rifles (Lee-Enfield Mk1 NoIII being the most common) and had the machinery in place to make sporting versions.
After the lions were killed, their skins were used as rugs by Col. Patterson. They were later sold to the Field Museum of Natural History in Chicago, who had them stuffed and placed on display. Because they were originally used as rugs with resulting deterioration of the hides, the lions are much smaller than they originally were.
The real John Patterson is credited with killing both lions on his own, and Val Kilmer's character John Patterson does indeed kill two lions on his own in the movie. The first is indeed a mane-less lion (as a Tzavo male lion might be, though not considered part of the Ghost/Darkness pair in the film) killed by Patterson on his first night in the encampment. The second is the second lion of the "Ghost/Darkness" pair. Patterson also wounded the first lion of the pair with a handgun and then (the fictional) Remington finished the lion off.