A number of iPhone X users have reported that they’ve been finding it difficult trying to catch Legendary Pokémon in Pokémon GO. The above image is a visual representation of the iPhone X useable screen area compared to other devices. This is the main reason as to why Kyogre is nearly impossible to hit on iPhone X.
KNOWN: In order to format Pokémon GO for the edge-to-edge screen of the iPhone X, rather than expanding the visible area, Niantic essentially zoomed into the accessible game play area, cutting off the sides on the left and right of that space.
PROBLEM: Catching any Pokémon, and particularly Pokémon as far away as Kyogre, iPhone X users essentially run out of screen real estate trying to swipe outside the Pokémon hit box far enough to hit a curve ball in the center of the screen. If, like me, you’ve had trouble catching Kyogre on an iPhone X, this is why, and I think it’s time we revisit this issue before iPhone X users throw away any more $$$ trying to catch this thing over the next month. Niantic’s method of formatting for the iPhone X, the expediency of which was surely appreciated by many users, lacks the necessary modifications required to keep the game functioning properly on the device. At present, all iPhone users are handicapped, and I hope Niantic can implement a fix before Kyogre disappears forever.
Source: The Silph Road on Reddit