Ultra Rage is a specific type of rage boost first used by Seventy-Five. This is a transformation/powerup to the current form, and it's 5x stronger. Xamo and all of his creations have it, as well as Viaton and possibly Shadora.
Ultra Rage causes the user to glow red, their body to become under a lot of pressure, and their aura to change. Skilled users can activate this at any point, though Seventy-Five's first usage of it took a while and caused him great stress.
The user's power multiplies by 5, but it leaves their body overwhelmed, though not as much as kaioken. It can also be used for defense, as it boosts all stats.
Omega Seventy-Five accessed this after snapping, but it seems that his God form also has the ability, the latter being able to obtain it at will, and carry it into his next forms. Before using this powerup on his God form, he used it on his Gigapad form for a very brief amount of time after getting angry at Soul Weegee.
Vileon possesses the ability since he's an Xamo creation and only used it once against Star-X and attempted to kill him in one hit. He quickly ran out of power after using it.
Rotu-4 possesses the ability since he's an Xamo creation and used it many times against the heroes. He exercises far better control over it than most users, even being able to block Viaton's absorption ability with it. Since the present Rotu-4 is currently alive and allied with the heroes, there's a chance he could perfect it.
Viaton absorbed Rotu-4 from the past and gained the ability. He immediately used it, but it overwhelmed him and he couldn't control its power. As such, he went over the "Deity Level Limit" and got sent to Universe 6.
Xamo used a perfected version of Ultra Rage against Universia and the heroes after time was changed. It's presumed that Xamo knew this in the original time as well, but he never used it then. Xamo can sustain the form for a lot longer than usual, as it doesn't use up much stamina, and the glow is less intense.
Shadora's ability to learn/copy techniques would lead to the belief that he can use Ultra Rage, as he's seen many people, even Xamo, do it. He has never used it, possibly because it could destroy his body.
Hecon was seen in a cryotube when Xamo released Rotu-4. He can presumably use Ultra Rage due to being an Xamo creation. He never used it.
Squinian was seen in a cryotube when Xamo released Rotu-4. He can presumably use Ultra Rage due to being an Xamo creation. He never used it.
Zamon was seen in a cryotube when Xamo released Rotu-4. He can presumably use Ultra Rage due to being an Xamo creation. He hasn't used it so far.