Udyr's "Turtle" stance should actually be "Tortoise" stance
First, the stance picture, from league wiki: http://vignette4.wikia.nocookie.net/leagueoflegends/images/4/46/Turtle_Stance.png/revision/latest?cb=20130929123410
Actual tortoise (one example): https://upload.wikimedia.org/wikipedia/commons/a/aa/A._gigantea_Aldabra_Giant_Tortoise.jpg
Notice the similar pronounced hump in its shell?
Whereas a turtle has a much flatter and smoother shell with no such hump: http://animalia-life.com/data_images/turtle/turtle3.jpg
Also in ancient Rome they had a defensive formation called the "tortoise" or "testudo" formation, not the "turtle" formation.
His spirit guard skin makes some references:
"Indestructible!" "Push through their defenses." "Give no ground." "Maintain guard." "Defend all sides." "Slow and steady." (Tortoise and the Hare) "The turtle is one part fortress, one part mobile home." (TORTOISE)
But again, there is no such thing as a "turtle" battle formation, only "tortoise".
Also, turtles are aquatic and tortoises are terrestrial (thought I should mention that as tigers, bears and even phoenixes all live primarily on land).
Also, they kind of flip-flop between tortoise and turtle for things (FYI), such as skins, quotes, skill images, etc.