Fred's Backyard is an interactive fiction game and was released in 1991 anonymously.
Review by David Welbourn (originally posted at Baf's Guide)
A bizarre little game, where you rescue Fred from a sea serpent who is terrorizing a puzzle-filled forest and hotel, and who can be repelled with square roots. Expect a two-word parser, primitive graphics, several math-based puzzles, and sudden death. Fortunate, then, that the game knows about "save" and "restore".
But the most annoying feature is that many puzzles don't stay solved; they have their own screen apart from the gameworld and are used like doors between locations. This means that you might have to solve some puzzles multiple times, just to get around. On the plus side, there is a helpful map, so you at least know where the trouble spots are.