Ok,
What you need to do is make sure not only do you have a faceset and charset for yoshi, but that you have a battle charset for him uploaded into your battlecharset directory.When you have all this, get into your rpgmaker2k3 and goto your database.Click on the "Battle Animations 2" tab.make sure you have changed your data max at the bottom left-hand corner of the screen.I prefer to set this at 40 or more.when this is done, select a blank slot under all the names to the left.When you do this you should see that all the animations to the right dissapear.DO NOT PANIC.
Now enter the name "Yoshi" under animation name toward the top middle.Rite below that, you should see three tabs marked "Weapon","Standart", "Extra". Click on standart.Now set all your yoshi battle animations as you will.Do this by clicking the "SET" button by where you whant to set one.Don't be lazy and try to find a differant way to do this.Of course it takes time,but what game making process doesn't?Well....I hope I helped ya.