This is missable but not too long from the beginning of the game if you did miss it. Once you get into the office in the gallery you start in, the goat is on the top shelf above the desk, on the left. I don't believe you get a notification to click the goat, just click around the top shelf and the achievement will pop.
There's a couple more opportunities to get the achievement but once you start episode 2 you would have to reload a save.