Games like Platypus II
Games like Platypus II include BALL x PIT, METAL SLUG, Planes, Bullets and Vodka, Touhou Koumakyou: New Classic - the Embodiment of Scarlet Devil, and Revolution Ace. We matched them by the tags players give Platypus II on Steam (Action and Shoot 'Em Up): closest matches first, and among equally close ones the better-known games.

BALL x PIT
Shared: Action · Shoot 'Em Up

METAL SLUG
Shared: Action · Shoot 'Em Up

Planes, Bullets and Vodka
Shared: Action · Shoot 'Em Up

Touhou Koumakyou: New Classic - the Embodiment of Scarlet Devil
Shared: Shoot 'Em Up · Action

Revolution Ace
Shared: Action · Shoot 'Em Up

METAL SLUG X
Shared: Action · Shoot 'Em Up

Clownfield 2042
Shared: Action · Shoot 'Em Up

Raw Data
Shared: Action · Shoot 'Em Up

Grand Theft Auto V Legacy
Shared: Action

Guns, Gore and Cannoli 2
Shared: Action · Shoot 'Em Up

Guns, Gore & Cannoli
Shared: Action · Shoot 'Em Up

Soda Crisis
Shared: Action · Shoot 'Em Up

Star Saviors
Shared: Shoot 'Em Up · Action

Cyberhunt
Shared: Action · Shoot 'Em Up

Savant - Ascent
Shared: Action · Shoot 'Em Up

Ikaruga
Shared: Shoot 'Em Up · Action

Touhou Kouryudou ~ Unconnected Marketeers.
Shared: Shoot 'Em Up · Action

Touhou Juuouen 〜 Unfinished Dream of All Living Ghost.
Shared: Action · Shoot 'Em Up

Crysis 2 Remastered
Shared: Action · Shoot 'Em Up

DRAINUS
Shared: Action · Shoot 'Em Up

Bit Blaster XL
Shared: Shoot 'Em Up · Action

ZeroRanger
Shared: Shoot 'Em Up · Action

Octogeddon
Shared: Action · Shoot 'Em Up

Nova Drift
Shared: Shoot 'Em Up · Action