In the prototype version of Yoshi's Island, the entire concept of Kamek creating the bosses by enlarging regular enemies has not yet been implemented, so the bosses are simply already large creatures to begin with.
This allows the game to do something that was impossible to do in the final version, which is to have the already large forms of the bosses appear before the battle starts. The footage shows the approach to the Naval Piranha boss battle in the prototype, where tendrils emerge from the water and lure Yoshi in before the actual boss appears. While not shown in the footage, the tendrils also hurt Yoshi if he touches them.
While it is unfortunate that this had to be removed as it would no longer fit the concept in the finished game, it was replaced with the equally memorable Easter egg whereby Yoshi can defeat Naval Piranha before Kamek can enlarge it, skipping the entire battle.
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