The HP Bar
The HP Bar is a formatting quirk used for appearances of particularly powerful Pokemon, especially signature Pokemon of particularly experienced Trainers.
In its most common form, the HP Bar is a series of Full Block Elements (█) in a row, often either in sets of 64 or 100, with color formatting around whatever section represents the owner's current amount of health. Usually it is aligned to the center of the page, rather than to the left. There are occasional variations, particularly in its first appearance during Jacob's fight against Pallad, in which it used forward slashes (////) instead of block elements.
It is unknown if the HP Bar actually has a physical appearance in-universe, although art of Pallad features it hovering over his head with the appearance of the bar based on how it looks in Pokemon Platinum.