At first, nothing will come out. Usually it will seem like it's just the boss, but that's completely wrong. Eventually, a Mooth will come out, soon followed by Hippoe. Take warning that this stage seems Veteran difficulty for early game users.
Just play it safe, and hold the peons off for a bit, and gather up money. When you're ready, overwhelm the Mooth and Hippoe with your units. The boss should appear shortly thereafter, which is a weakly buffed Blitzen. It's really powerful for early game players, so don't underestimate it. You'll want to use tanky units like Mythical Titan Cat, or strong ranged Cats, like Sexy Legs Cat or Dragon Cat.