From the Super Mario Wiki
Baseball Boys are a sub-species of Shy Guys in Super Mario World 2: Yoshi's Island with clear relation to the Zeus Guy and Bandit. Aside from the physical similarities to those two sub-types, Baseball Boys specifically operate using baseball equipment. They often appear in groups of two or more, with two different types observed.
Grunts also greatly resemble the members of the Baseball Boys and may act as umpires for them. Also, as stated above, they bear resemblance to the Zeus Guys and Bandits (who very likely have no direct relationship to them).
Baseball Guys are similar to Chargin' Chucks, who appear in Super Mario World, in some respects. Both groups appear to be athletes of an American sport, and both take three jumps to defeat. Also, Yoshi can't use his tongue against the two groups either.