First introduced in the original Dragon Ball, the Oozaru form native to all Saiyans was a destructive– nearly uncontrollable– form that turned them into bloodthirsty Great Apes at the sight of the full moon. It was also a source of pain for Goku, who later in life discovered that he was the one who killed his grandfather while rampaging in this form as a child.
While the Great Ape bears a few similarities to modern werewolf lore, it is just unique enough from its influence to stand out among the many, many other transformations shown in the Dragon Ball series, though not always because the form makes any sense.
10 Destroying The Moon
Part of the transformation process into a Great Ape form requires the subject to look directly at a full moon, triggering the chemical reaction that begins the transformation process from the tail. While cutting off the tail makes some sort of sense to revert a Great Ape back to a regular Saiyan, destroying the moon, which has been done multiple times, also seems to do the trick despite the fact that the moon's involvement has already come and gone by that point. That's also not taking into account the global ramifications for the moon suddenly disappearing.
9 Blutz Waves
During the Saiyan arc, Vegeta stats that the main catalyst for transforming into a Great Ape is blutz waves, rays of sunlight that change certain properties as they reflect off of the moon's surface. There is no reason given as to why regular sunlight doesn't sometimes inadvertently trigger a transformation, since blutz waves are technically a form of sunlight. This is one case where trying to explain something in too much detail raises more questions than actually answering them.
8 Differing States Of Control
During Vegeta's assault on Earth, he states multiple times that there is a clear divide between him and Goku, though most of it is nonsense associated with his ego and high born status as an Elite Saiyan. Unlike Goku and Gohan, Vegeta was in total control of his Great Ape form, though there is no reason given as to why aside from Vegeta's own inflated sense of himself.
Even when it is established that Goku and Gohan are more powerful than Vegeta, as well as being capable of advancing far above his own power, neither of these protagonists have been able to control their Great Ape forms, even in the non-canon Dragon Ball GT.
7 Vegeta's Armor
One of the most recognizable outfits in the Dragon Ball series is the Saiyan armor, otherwise known as the armor of Frieza's army. During Vegeta's assault on Earth, it was shown to be able to stretch to incredible sizes while still retaining its shape in his Great Ape form, though later in the series it also shows that it isn't as pliable as it once was, even cracking and breaking when hit with ki blasts.
6 Super Saiyan
After the events of the Saiyan arc, Vegeta becomes laser-focused on the idea of Super Saiyans, legendary warriors who can make even the strongest of their race seem like low born cannon fodder in comparison. While pretty much every Super Saiyan form is based on the same kind of transformation, it begs the question as to why it is so radically different from the Great Ape form. It also doesn't make much sense that a race would have two very different forms of transformation biologically linked to their species with very different ways of activating said transformations.
5 Golden Great Ape
The Golden Great Ape, though not canon to the Dragon Ball universe, has the signature golden fur associated with the Super Saiyan transformation, as it is basically a Super Saiyan Great Ape– but there is one big difference that doesn't make sense in context. Aside from the golden hair synonymous with a Super Saiyan, their eyes also turn bright blue as opposed to their usual black color. With a Golden Great Ape, their eyes stay the bright red that is a part of their normal Great Ape forms, not changing color despite the added transformation.
4 Mouth Beams
For the most part, characters in Dragon Ball will use their hands to fire ki blasts, as it allows for more accurate shots and greater control of their power. There are exceptions to this, with Piccolo's eye lasers being a prominent one, but the mouth beams are one that come up a few times with villains of the series as well as the Great Ape forms.
In particular, Gohan was shown using them repeatedly during one of his Great Ape rampages, even though he was barely able to control his ki in his base form at that point and had no control over his Great Ape form at all. This means that either Gohan's Great Ape form has more control over ki than his base did, or the animators just thought it looked cool.
3 Cell
As he is made of the DNA from many Saiyans, it stands to reason that Cell could have transformed into a Great Ape given the right conditions, particularly since he has a tail. As he had intimate knowledge of the Dragon Teams' techniques, it is likely that Cell could have made use of this transformation during the Cell arc, as he was capable of using Piccolo's regeneration technique despite only having the cells from one Namekian. Though this is unlikely to happen given Cell's obsession with his perfect form, it could have made for an interesting battle with the Ascended Saiyan form of Gohan.
2 Goku's Secret Transformations
Goku may have only been a child when he first transformed into a Great Ape, which explains why grandpa Gohan never told him about it, but that doesn't explain why Bulma, Yamcha, Puar, and Oolong didn't tell him about it. By this point, he was much older, though still naive, and could have been able to understand what had happened. Instead, Goku doesn't find out until Dragon Ball Z, when he finally puts the pieces together and realizes that he was the one who killed his grandfather.
1 The Whole Transformation Process
During the Saiyan arc, Vegeta explains that the transformation process involves a Saiyan absorbing blutz waves through their eyes, which starts a chemical reaction in the tail and transforms them into a Great Ape. While this was designed to explain how Saiyan physiology works, it ends up raising more questions than it actually answers, particularly when Vegeta's fake moon technique comes into play. He had just said that blutz waves were formed by sunlight reflecting off of the moon, then says that the same result can be achieved by a reaction in the upper atmosphere instead. Then there is the fact that Gohan had previously transformed when a hologram of the moon was projected into the night sky while training with Piccolo, making the whole process nonsensical and inconsistent.