Horton hears a who was actually a pretty good movie, I enjoyed it alot.
They probably feared it could be too successful when they made these awful movie posters, probably with the intent to scare kids away.
For comparison, here's real Horton:
And here, as characters on movie posters always have to look hardcore:
"Ain't I cool as I'm about to step on this bridge which could cost me my life?"
Here's the same from the movie with an appropriate expression: