Answer
See below.
Work Step by Step
Step 1. To obtain the graph of $f(x)=-e^{x}$ from $y=e^{x}$, reflect the curve across the x-axis. See graph.
Step 2. We can determine the domain is $(-\infty,\infty)$; the range is $(-\infty,0)$; and the horizontal asymptote is $y=0$.