There's one I forgot to include for the Atari ST. It was originally on there but there are also ports for PC and Amiga. It is, of course, Llamatron.
Designed by a legendary weirdo called Jeff Minter and released as freeware, Llamatron starts with the arcade twin stick shooter Robotron, turns the player into a llama firing bullets from its eyes, turns the enemies into random objects like Coke cans, packets of Rizlas, cow heads, smiley faces, potplants, and so forth, and includes a number of "beasties" such as goats, camels, yaks, and other ruminants that are to be rescued for bonus points, includes ridiculous sound effects, seizure-inducing inter-level graphics, and adds a variety of other special monsters such as giant 16 ton weights that drop on you from the top of the screen, indestructible laser beams, toilets that spew bog rolls with every flush, blue brains that turn beasties into evil "zombeasts," and a glowing mandelbrot set called Screaming Mandy that chases you and makes ear-rape noises when shot.
It has to be seen to be believed.
Oh, and there's apparently been an Android port as well. Ohhh yeahhh!
https://youtube.com/watch?v=y4i0y2NMq40