Here is an explicit formula for your ratio $r_n=\frac{n_n}{d_n}$:
$$r_n=
\frac{\sum_{k=0}^n\binom{n+k}{2k}(-x)^k}
{\sum_{k=0}^n\binom{n+k+1}{2k+1}(-x)^k}.$$
Let $P_n(x)$ and $Q_n(x)$ be the numerator and denominator polynomials of $r_n$, respectively. Then both polynomials share a common recurrence; namely,
$$P_{n+2}+(x-2)P_{n+1}+P_n=0 \qquad Q_{n+2}+(x-2)Q_{n+1}+Q_n=0.$$
They differ only in the initial condition where $P_0=1, P_1=1-x$ while $Q_0=1, Q_1=2-x$. The importance of such a description is that (1) the original recursive relations are decoupled here; (2) it is more amenable to an asymptotic analysis; (3) it reveals the roots being in $[0,4]$ due to the **interlacing property** of three-terms recurrences.

**Note**. The original numerator and denominator differ by $\pm$ sign from $P_n$ and $Q_n$, but this makes no difference for the ratio $r_n$.