It's kind of funny to see early concepts of Tigger, before the final design was set.
This is the cover of sheet music for the Sherman song "The Wonderful thing about Tiggers"
You can see that Winnie the Pooh is finalized, he looks like he appears in the classic short films.
Tigger not so much. Milt Kahl must have been busy with another assignment, he certainly had nothing to do with this temporary design, or the way story artists depicted the bouncy character.
There is a looong way from these unrefined concept designs to what Milt finally came up with.
Milt loves to draw some of his characters with an underbite mouth configuration. Madame Mim, Shere Khan, Bagheera and the Fisherman Bear from Bedknobs & Broomsticks to name a few.
Tigger is no exception. In this case it adds personality and looks more interesting, graphically.
One of Marc Davis' favorite Disney Characters.
More infos on the development of Tigger in this previous post: