While Super Saiyan is iconic for Goku, no form fits Dragon Ball's Vegeta more than Ultra Ego. Despite how the ultra-powerful transformation seems tailor-made for the Saiyan prince, a series of old sketches from series creator Akira Toriyama reveals it shares more than a passing resemblance to Goku's original design for Super Saiyan 3.
After Goku unlocked the silvery-tinged Ultra Instinct form, fans were itching to see what Vegeta's equivalent would be. Their prayers were answered in Dragon Ball Super's 74th chapter where Vegeta debuted Ultra Ego against the universe's strongest warrior, Granolah. While Vegeta eventually lost that fight, the form left a mark on fans as it's such a perfect fit for Vegeta. Unlike every other transformation he can access, Ultra Ego is completely unique to Vegeta. With its ability to absorb damage and then dish it right back, there's no doubt that Ultra Ego is objectively Vegeta's strongest form. It's ironic then that the basis for Ultra Ego's design is eerily similar to one that first belonged to Goku.
In Dragon Ball - 30th Anniversary Super History Book fans of the global sensation are given a closer look at some of series creator Akira Toriyama's initial sketches for character designs. Eagle-eyed readers can spot two early prototypes of Super Saiyan 3. One of them looks similar to the current Super Saiyan 3 design with the sloped brow and ridiculously long hair, though this early design also incorporates a tail similar to Super Saiyan 4. Of far more note is the design next to it, which gives Goku a sloped brow and blown-back spikey hair. It's difficult at first to tell that it is Goku at all at first because this early version of Super Saiyan 3 resembles Vegeta and his later Ultra Ego form so closely. However, given that Vegeta never goes Super Saiyan 3 in the original Dragon Ball manga, and that this design lacks Vegeta's trademark widow's peak, the design is definitely Goku.
While the two designs are remarkably similar, it is almost assuredly a coincidence as opposed to Toriyama recycling any ideas. As a matter of fact, Ultra Ego wasn't Toriyama's creation at all. In an interview (released on Twitter by Dragon Ball fan account DCSChronicles), Dragon Ball Super's artist Toyotarou explained his process for creating Ultra Ego and getting it approved by Toriyama. In that interview, Toyotarou doesn't mention any inspiration from this early Super 3 design, basically confirming that their similarities are a happy accident.
At the end of the day, it's for the best that Toriyama went with the long-haired Super Saiyan 3 form that fans are familiar with. It is far more visually distinct from Super Saiyan 2, than this original design. Still though, it's tempting to wonder what Vegeta's Ultra Ego would look like if Dragon Ball had used this original design for Goku's Super Saiyan 3.
Source: Dragon Ball - 30th Anniversary Super History Book, DBSChronicles