I would also agree with DeathLancer's assessment that it was an entity drawn by the opening your spell left. In Wicca, the Law of Attraction or "Like attracts Like" is often used to describe how entities will be drawn to a given working.
When I was in Elementary School, a friend and I found this book, " Black Magic, White Magic " or perhaps the reverse of that name, we never found it in the School's Library since it was returned. We had a sleepover at my house and decided it'd be fun to play around with a spell from the book. We decided to "Summon a Vampire".
We didn't have any supplies, we just read the evocation. While reciting what was on the page, we heard a jingling tune. I looked over my shoulder to the bookcase and saw the music box that was there opening up and playing the tune that it held.
Needless to say, he and I both looked at each other with an, "Oh Shit!" expression. We didn't actually say that, since it was a year before cursing became the "cool thing to do". But we stopped. About this time, the storm that was already on its way started thundering and as we were looking out the window at it, we could have sworn there was a wolf looking at us.
No vampire showed up, but we were quite disturbed by the experience.
Did something malevolent come? I doubt it. It was probably some entity that saw two little boys that needed to be taught a lesson. I doubt anything in the book would have worked. It was probably called by the intent behind the recitation, more so than the actual words. We left an opening, and it knew we weren't ready for something worse if we decided to start developing darker intentions.
The fear of pain or bad happenings is often worse than them occuring. "What If" is a powerful thing, and the human mind can concoct a lot worse than could be done to us. I suspect this is true from what happened to you, FatRaspberry.