We investigate the strong-coupling regime of a linear x-x coupled harmonic-oscillator system by performing a direct diagonalization of the Hamiltonian. It is shown that the x-x coupled Hamiltonian can be equivalently described by a Mach-Zehnder-type interferometer with a quadratic unitary operation in each of its arms. We show a sharp transition of the unitary operation from an elliptical phase rotator to an elliptical squeezer as the coupling gets stronger, leading to the continuous generation of entanglement, even for a significantly thermal state in the ultrastrong-coupled regime. It is also shown that this critical regime cannot be achieved by a classical Hookian coupling. Finally, the effect of a finite-temperature environment is analyzed, showing that entanglement can still be generated from a thermal state in the ultrastrong-coupled regime but is destroyed rapidly.