Thunder

a horse frightened by a thunder

Lightning is similar to thunder. Due to sound being slower than light, we see the lightning before we hear the thunder

Thunder is the noise caused by lightning. It occurs because the light from the beam causes the air to heat up very quickly and as it increases in size it causes an explosion.

Although the loud and rumbling sound it makes can scare us, in general thunders are harmless, since we hear them after the lightning has already struck.