Google Assistant could make it onto non-Pixel phones according to some recent findings in the latest APK for the alpha version of the Google app. While alpha releases of Google apps aren't generally available, at least for the Google app itself, beta versions of the Google app and other apps that Google offers are available for installation. In regards to the Google app, some users who are part of the beta channel have been getting an alpha release build.
While this is a little bit out of the ordinary and it isn't clear if Google had intended to send these builds of the app out to users, the more intriguing detail is that Google Assistant becomes available on devices which aren't a Google Pixel after installing the alpha release of the Google app. What's more is that Google Assistant is reportedly working for users who have this Google app build. According to the details it's been stated that it's working on both Android 7.1.1 Nougat and Android 6.0.1 Marshmallow. The Nougat-powered device was a Nexus 6P, which may not seem as strange since it is a Google device, but the device running Marshmallow is a Samsung Galaxy Note 5, which suggests that Google may be planning to roll out Google Assistant to other non-Pixel and non-Nexus devices.
It's also possible that Google Assistant won't require users to have Android 7.0 Nougat installed, though it may be required to have at least Android 6.0.1 Marshmallow for Google Assistant to work. It's also stated that both of these devices were able to access Google Assistant by holding down the home button, which could very well be the way other devices access it if Google rolls it out widely with a stable release of the Google app in the future. It's noted that not all devices which have installed the alpha build of the Google app have had Google Assistant appear when holding down the home button, but just the fact that it's appearing is likely to be exciting for users who have wanted access to the feature but don't have a Pixel or Pixel XL to utilize it. When Google might plan to make the feature available with the Google app officially is unclear, but at least it seems to be on the way, and possibly soon.