From the Super Mario Wiki
Gobblins are large, yellow, smiley face balls that appear in Yoshi's Island DS. Unlike most enemies, they cannot hurt Yoshi if he touches one, instead bouncing him up or back, making them more of an obstacle than an enemy. However, they are always seen blocking Warp Pipes, making it necessary to defeat them to gain access to the pipes.
In order to do so, the player must throw eggs at the Gobblin, which it proceeds to eat, growing bigger with every egg. After being fed five eggs, the Gobblin starts to look sick, at which point only one more egg is needed to make it explode. In the Island Museum, these creatures appear in the Subterranean Enemy Exhibit, and are one of the few defeatable enemies in the museum.