Llamatron: 2112 (or simply LLamatron) is a shoot 'em-up created by Jeff Minter and released as shareware in 1991 for Amiga and Atari ST. Later it was also ported to MS-DOS.
It is, as you can expect, a clone of Robotron: 2084, except that the player controls a Llama and he has to save other animals from strange alien enemies. You can play with a friend, or you can ask the computer-controlled droid to help you. Fire is automatic, but if you hold the button, you will continue firing in the same direction while you can move with your llama. Plenty of powerups, such as love potions, new weapons, and bombs, will help you in your quest.
This clone of Robotron is one of the most popular non-commercial games ever created for the Amiga and Atari ST, and it's still one of the most played today. You should not miss it.
One last thing, if you want to enjoy fully the digitized sound effects, add the word "FUCK" to your name when you enter the hi-score, then restart another game. Enjoy!