Combining insulating and metallic ferromagnets in hybrid heterostructures adds novel functionalities by harnessing the virtues of both material systems [1,2]. Here, we use bilayers of yttrium iron garnet (YIG) and nickel (Ni) and fabricate nanowire devices using e-beam lithography and ion-milling. The magnetoresistive phenomena of Ni allow us to access static and dynamic states of the heterostructure [2-4]. We investigate hybrid magnons with magnetic field applied perpendicular to the device plane and identify aligned and non-aligned modes. By supplying electric currents, we observe microwave emission from the nanowire, corresponding to auto-oscillations of the non-aligned mode. The signal is symmetric in both field and current polarity , indicating an excitation by thermal spin currents. Such thermally-driven auto-oscillations in heterostructures with noncollinear magnetization open avenues for spin-orbitronic functionalization of hybrid magnetic systems and would benefit neuromorphic computing  and quantum magnonics.  Safranski et al., Nat. Commun. 8, 117 (2017);  Arkook et al., arXiv:1909.12445;  Gonçalves et al., Sci. Rep. 8, 2318 (2018);  Navabi et al., Phys. Rev. Appl. 11, 034046 (2019);  Etesamirad et al., ACS Appl. Mater. Interfaces 13, 20288 (2021) *This work was supported by the National Science Foundation through Grant No. ECCS-1810541.