Game File: 

Gavin on Mars.zip

'A monkey named Gavin has to ride a horse or a unicorn into a castle. Gavin lives on Mars and wears a crown. He is playing games and stuff in the castle. The name of the game is "Gavin on Mars." (Courtesy Eric, age 4)'

A Kickstarter request. I tried to keep the difficulty down in the hope that Eric might play it. There's a wee dedication to him if you press the 'E' key on the last screen.

Gavin on Mars: Eric Expansion Pack
Eric, and SpindleyQ, have since expanded on the game, which you can get here: http://www.glorioustrainwrecks.com/node/2833