When I started meditating back in 1976 I had some scary experiences - I think that when beginning to meditate and calm the mind that some of our inner turmoil begins to surface in pictures that evoke distress to us - hence your skull picture. And if you were feeling suddenly scared your dog probably was responding to the hormones of fear you were exuding at the time.
If that happens again, stop the session and write out the scary episode in a journal - perhaps there is something going on in your life that wants to surface and be resolved. Just a thought. <img src="/images/graemlins/smile.gif" alt="" />
You don't need to focus on one thought in meditation. Try counting your breaths in and out - count 1,2, breathe in; count 3, 4 breathe out; count 1,2 breathe in and continue.
Even breathing helps calms both mind and body and in this body-mind harmony we feel our greatest peace.
Blessings, Susan