Step 1 :Given that $f$ is a monomial function such that as $x \rightarrow \infty, f(x) \rightarrow -\infty$. This implies that the function must have an odd degree and a negative coefficient.
Step 2 :By substituting a large value of $x$ into each function, we can determine which functions tend towards negative infinity as $x$ tends towards infinity.
Step 3 :For $f(x)=-\left(\frac{1}{7}\right) x^{4}$, $f(x)=-3 x^{2}$, and $f(x)=-x^{9}$, the results are negative for a large $x$, which means they tend towards negative infinity as $x$ tends towards infinity.
Step 4 :For $f(x)=5.5 x^{5}$ and $f(x)=x^{10}$, the results are positive for a large $x$, which means they tend towards positive infinity as $x$ tends towards infinity.
Step 5 :Therefore, the function formulas for $f$ could be $f(x)=-\left(\frac{1}{7}\right) x^{4}$, $f(x)=-3 x^{2}$, and $f(x)=-x^{9}$.
Step 6 :Final Answer: \(\boxed{f(x)=-\left(\frac{1}{7}\right) x^{4}, f(x)=-3 x^{2}, f(x)=-x^{9}}\)