From Snopes:
An alternative theory holds that the creature's altered appearance in taxidermy form was intentional and/or the result of the taxidermist's expectation that the animal would be viewed only from a particular angle:

When seen from the side the lion actually has quite a ferocious appearance. In this context, the lion’s bizarre tongue and fake teeth make some kind of sense. Even the close-set position of the eyes seems to have been done in order to make them appear furrowed and angry. Perhaps the taxidermist simply focused too much on side-view images when mounting the pelt, or the lion was (somehow) never intended to be seen from more than one angle.

Here’s a picture from the side.