The animal hormone which makes the heart-beat faster in a scary situation is Adrenaline.
The hormone adrenaline is usually secreted by both adrenal gland and certain neurons. It plays a major role in the "fight-or-flight" response by increasing flow of blood to muscles along with the output of the heart, which leads to pupil dilation response and blood sugar level. In layman terms it provides a jolt of useful energy and simulation.