Genson! Kodai Seibutsu-shi Packy

One day, elementary student Takeru meets a very strange new friend-- a Pachycephalosaurus named Packy! He's traveled from "The Lost World," a place where extinct animals live together. Packy loves head-butting things, and he's traveling to discover the perfect head-butt target. Now Packy may end up living with Takeru, his older sister, and his parents in their modern-style home. However, Packy is not the only creature to come out of the Lost World, and other creatures may have much less friendly intentions toward the modern world and humanity!

(Source: MangaHelpers)