Unused Stuff
It seems that more was planned for this game, judging by the abundance of unused assets.
--Graphics--
Unseen frames
For every colored Yoshi, the second frame of the xhit animation(x being the swedish initials for the Yoshi colors) is never seen due to the game sleeping for 1000 milliseconds and then restarting the room.
How it would look like if the sleep action and the rest of the actions is removed from the enemies' collision event with the player:
Win sprites
All Yoshis have sprites for clearing a stage. A unused object called superstar would have changed the player sprite to this graphic when touching the object. Unfortunately, getting all the eggs just spawns the star object, so this graphic won't be seen anyway. Due to the object coming after the World 5 objects, it is possible it may have been meant for beating the last boss. Even so, it's just a theory.
Larger versions of 4 of these sprites appears as part of the background in the Yoshi color changing screen, but there they are tiles instead of sprites.
Bigger versions of the Yoshi eggs exists within the sprites. It is unknown what these sprites would have been used for.
Larger versions of the unused xwin sprites.
Unused Yoshi colors
Some Yoshi colors are not used in the game, namely blue, purple, black, and white. The presence of these colors, as well as their respective eggs in the levels could suggest they were planned to be playable at one point.
Unused items
Several sprites suggests that additional items were going to be in the game; melon, chili, grapes, banana, coin, redcoin, and extralife. The banana makes an appearance in a screenshot of Yoshi's Egg Hunt 3: Search for the happy tree that is shown in the embedded wma file, but there it looks more simplistic. None of these graphics have any objects defined, but removed object IDs 62 to 68 may have been reserved for these items at some point. Aside from the extralife sprite, what purpose the other items might have served is unknown.
flare
This is identical to the image shown while the game loads. But why is this even included with the sprites? We may never know. Appropriately, this is the last sprite in the sprite ids, as no other graphics exist beyond this.
Graphical Oversights
When playing as any Yoshi other than the green one, the stars that appear above Yoshi when hit are colored differently. This also applies to the unused Yoshi colors. They were probably left alone when making the colored variations of Yoshi, though it does look a bit odd. The arms/hands are also mostly transparent.
The second frame of the Yoshis' left and right run animations have a transparent portion on the belly. This probably happened when redrawing the first frame to move the left/right shoe.
demo
This background image is probably leftover from a demo version of the game.
sky
Several non-level screens use this background, but it is set to be invisible, and only consist of a single color.
--Sounds--
hurry.mid
Unknown where or how this would've been used. Probably levels were going to be timed at some point in development.
story.mid
Identical to mellan ghost.mid. Maybe a story intro of sorts was planned at one point?
final!.mid
Probably intended for the final boss? It's possible that it went unused due to the short length.
Explosion.wav
Standard sound effect taken from the files provided with Game Maker.
1up.wav
sound for collecting a 1up. The game doesn't use any lives system. Getting hit just takes away one point from the score, and there's also no way to get any game over.
enemy.wav
boss.wav
superstar
Similar to the used object star, but it also sets the player sprite to gwin. This applies to object green, and THAT object only. This object also handles things like setting the score and going to the next room(with the latter having no transition effect set).
Amusing variable name
In each of the Yoshi objects, the variable name for the egg count is bajs("bajs" being the swedish word for "poo"). Not really unused, but it is something that's always sounded rather funny to me, and unless one has got cheat engine or some other kind of process hooking program, most players would probably never see this variable name anyway.
invalid resource names
Several resources like sprites, sounds, objects, and rooms are either named identically or have plain invalid names. It would only have been a problem if objects in the game used code. Since objects uses drag and drop actions, it has no ill effect on things in the game.