Mantine

Mantine definitely exists.

Its attack is too low to make a dent in raids, and its Water/Flying typing means it will be absolutely fried by any gym attacker packing an Electric Type move for gym defense. It's probably best to pass on this flying manta ray. 

Best PvE Offensive Moveset

• Bubble pairs well with Water Pulse for Water type damage. 
• Wing Attack pairs well with Aerial Ace for Flying type damage. 
• Bullet Seed doesn't perform as well, TM this.
• Ice Beam can provide coverage in niche situations.

Best PvE Defensive Moveset

• Bubble is Mantine's best defensive fast move with superior damage and energy gains. 
• Wing Attack and Bullet Seed don't perform as well as Bubble.
• Ice Beam has an early damage window and can provide a wide coverage.
• Aerial Ace costs the least amount of energy and is more likely to be used before Mantine faints. 
• Water Pulse has a longer damage window but can help cover Mantine's weakness to Rock types. 

Bubble generally possesses the best balance between energy and damage. Wing Attack, after the nerf in Pokemon GO: Max Out, is generally a more niche option to hurt Grass-type Pokemon more reliably. Bullet Seed is a decent move overall due to its high energy gain, but its very low damage output makes it not worth using, especially as Mantine's Charged Attacks aren't particularly damaging.

Aerial Ace is Mantine's fastest move and deals reliable damage. Ice Beam allows Mantine to defeat other Flying-type Pokemon. Water Pulse is a rather middling move, but it benefits from STAB and gives Mantine coverage against Steel-type Pokemon. Bubble Beam can be used to apply debuffs, but it is very weak.