Here are some that spring to mind:
Growing up, I was fascinated by Hugh Lofting's pushmi-pullyu—kind of a gazelle, bi-horned unicorn cross:
from The Story of Dr Dolittle (1920)
Lewis Carroll's fantastic nonsense creatures came to life in John Tenniel's exquisitely detailed drawings:
The gryphon, which looks to be a lion, raptor, not-sure-what-else kind of critter; and the mock turtle— a wonderful blend of cow and turtle!
from Alice's Adventures in Wonderland (1865)
. . . and the ultimate composite Alice creature, the jabberwocky:
from Through the Looking glass, and what Alice Found There (1871)
On a more modern, and lighter note, here are a few jumbled-up characters from Dr. Seuss, complete with a variety of miximal-type names:
from If I ran the Zoo (1950)
Finally, I think Maurice Sendak's Wild Things are some of the best "miximals" going!
from Where the Wild Things Are (1963)
What are your favorite miximals from children's literature?