It was a game from Broderbund for the Atari 800 on cassette tape back in 1981. I can't remember the exact title but I remember it was at the Big Ben's music store on Ventura Blvd. in Encino. Back in those days you had to drive a ways to find the stores that handled computer games.
I can't find a listing through Google but I believe the word 'lunar' was in the title. The game was a variant on the 'Lunar Lander' genre in which you had to land on one of three locations with a stranded astronaut so he could scamper into the ship. The major complication was a big lizardoid monster that would show up. If you landed on the opposite side of the monster from where the astronaut was, he would still try to run to your ship and leap into the monster's mouth en route. The monster had some cute masticating animation that would cause us to feed it repeatedly instead of playing the game correctly. Big novelty value back then.
You didn't have any weapons except for your ship's thruster. If you hovered over the monster close enough it would be killed but this expended fuel, of course. I could probably put together a clone with immensely better presentation in Flash over the course of an afternoon but it wouldn't have the same specialness as it did.
After that I got a few other cassettes and cartridges. Shamus, Star Raiders, Missile Command, Centipede, Galaxian, Miner 2049er, and a few others. Then I fell into the world of piracy and ruined my appreciation of game purchases for years.