WARNING: The following contains spoilers for Chapter 129 of Attack on Titan, by Hajime Isayama, Dezi Sienty and Alex Ko Ransom, now available in English from Kodansha.
Thanks to the "curse" of Ymir, the first Titan, Attack on Titan's Nine Titan shifters are powers passed down generational lines at least once every 13 years. While shifters like Eren and Zeke have retained their original Titan powers throughout the course of the manga, in the case of the Jaw Titan, the changeover process has happened frequently, leading to significant changes in the Titan's appearance over the years.
In Chapter #129, the Jaw Titan once again gains a brand new form when its current holder, Falco Grice, transforms for the first time. And it doesn't look like any Titan we've seen yet.
We were first introduced to the Jaw Titan through Ymir (the other Ymir). During the Clash of the Titans arc, the Survey Corps member is forced to expose herself as a Titan shifter when she and her friends become stranded atop Utgard Castle amid an onslaught of hungry Titans. Ymir's Titan form was relatively small compared to the other Nine, with long, straggly hair, a haunched body and sharp nails and teeth. Her successor, Porco Galliard, one of the Marleyan Warriors raised to become a Titan shifter, looked more like the form's namesake: sporting a strong, armored jaw and a much more muscular physique.
Falco Grice, though a Warrior candidate, was never meant to inherit the power. After being turned into a Pure Titan, Falco ate Porco in Chapter #119 and inadvertently absorbed the Jaw Titan ability. The wild scramble in the wake of Eren triggering the rumbling has left no opportunity for the young soldier to learn how to it, which, as Pieck (the Cart Titan) points out in the newest chapter, never goes well on the first try. But, as the Eldian/Marleyan alliance battles the Jeagerists at the Paradis docks in an effort to stop Eren, Falco feels compelled to come to his allies' aid -- particularly when he sees Reiner (the Armored Titan) and Annie (the Female Titan) being battered to the point of immobilization by the enemies' Thunder Spears.
Falco's Jaw Titan form strangely seems to have taken inspiration from his own name, with a beaked mouth, feathered arms and talon-like fingers. This isn't the first time we've seen an animal-themed Titan: Zeke's Beast Titan form is very on-the-nose in its simian design. However, it's peculiar that one of the Nine's powers would factor its owner's name into its genetic components. Despite his unique look, Falco behaves typically for a newly-remade shifter -- quickly losing control and attacking poor Pieck.
As we've seen in Eren, Titan forms aren't set in stone; they can evolve and adapt. In time, could Falco take his avian design a step further and achieve flight? That would certainly give the group a quicker way to catch up to Eren's Titan army who are currently marching across the continent. Unfortunately, because of this, time is something they have precious little of.