In the files of Super Mario Sunshine, an unused Yoshi egg generator object can be found. It has no associated code, but based on its name, it would generate Yoshi eggs (the same way other generators in the game work, such as a Stu generator generating Stus).
Interestingly, there is also a purple version of it called “bad_gene_egg”, presumably standing for “bad egg generator”. While no remnants of its functionality are found, the fact that the Yoshis on Isle Delfino are stated officially* to be creations of Bowser Jr.’s paint, a “bad Yoshi”, i.e. an enemy Yoshi, could theoretically exist since this is the same paint used to create the enemies in the game.
*original 2015 Japanese version of the Super Mario Bros. Encyclopedia