Here are some links to the works of wildlife illustrators that I admire.
John James Audubon - the Haitian-born self-taught naturalist-painter who made "wildlife art" a thing. Original editions of his "Birds of America" are among the most valuable books in the world.
Edward Lear is primarily known as the writer of nonsense verse such as "The Owl and the Pussycat", but he was also an excellent and sensitive painter of wildlife (especially birds) and landscapes.
Ernst Haeckel - scientist, philosopher, and illustrator - produced beautiful art-nouveau-inspired plates of sea creatures for his book Kunstformen der Natur.
Beatrix Potter is, like Lear, mainly known for her children's literature ("Peter Rabbit"), but was also an exceptional wildlife illustrator, as well as a committed amateur scientist (she was among the first to realize the symbiotic nature of lichens, for example). (no link; I couldn't find a good site with a large variety of her scientific illustrations all in one place)
Charles R. Knight will be familiar to anyone who's interested in extinct wildlife - his murals defined paleoart for generations. Less well known but equally splendid are his many paintings of modern wildlife.
Louis Agassiz Fuertes - a wonderful bird illustrator, he also painted Native Americans and their artifacts as he accompanied expeditions across the US and around the world from the turn of the twentieth century until the 1920s.
Roger Tory Peterson invented the modern field guide in 1934, and made natural history more accessible for millions. While the illustrations in his field guides are necessarily simplified sketches, in his other work he showed a mastery of tone and composition rare among wildlife artists.
David M. Dennis has produced some gorgeous amphibian illustrations.
Joseph Tomelleri is the preeminent illustrator of North American freshwater fishes.