first off, YES, I know Brontosaurus didn't technically exist, and it was a mistake based on putting the wrong head on the wrong body, but consider this!

Equestria is FULL of mix-and-match creatures/races.

Unicorn- Kiang/narwhal/mammoth/yak/chiru/rhino/walrus/orangutan = 8 animals (I know a LOT about the mythological background of the unicorn)

Pegasus- Bird/horse = 2 animals

Alicorn- Unicorn (Kiang/narwhal/mammoth/yak/chiru/rhino/walrus/orangutan)/Pegasus (Bird/horse)/Earth Pony = 11 animals

Bat Pony- Bat/Pony = 2 animals

Manticore- Lion/Bat/Scorpion = 3 animals

Dragon- Lizard/Alligator/bat/wolf = 4 animals

Changeling- horse/insect = 2 animals

Breezy- horse/insect = 2 animals

Minotaur- bull/ape/goat = 3 animals

Centaur-Minotaur (bull/ape/goat)/horse = 4 animals

Griffon- Lion/Eagle = 2 animals

Discord- Alicorn (Unicorn (Kiang/narwhal/mammoth/yak/chiru/rhino/walrus/orangutan)/Pegasus (Bird/horse)/Earth Pony))/Manticore (Lion/Bat/Scorpion)/Dragon (Lizard/Alligator/bat/wolf)/Goat/Peryton (Wolf/Deer/Eagle)/Griffon (Lion/Eagle)/ Asian Dragon (Camel/Clam/Tiger/Carp/Rabbit/Snake/Cow/Eagle) = 32 animals

So if these exist, why not an Apatosaurus with the head of a Camarasaurus?

4317208 You may wish to read this, if you haven't already...

4319983 *Silently lamenting how far behind in paleontological news he has fallen*