In regards to the unidentified machine in the Halo 5 Trailer, these are my thoughts on it being a Forerunner War Sphinx.This, with reference: The War Sphinxes were described as being ten meters high and twenty long, with an elongated tail that provided lift and power. From the "tail" rose a large rounded torso with a command cabin perched on top. The command cabin was designed to resemble a Forerunner face, with features arranged in an angry or stern expression and two viewports serving as "eyes"is shown to give the unknown Forerunner Machine matches with the description of a War Sphinx; The "tail" is obscured by dust, obviously, but the "Feature arranged in an angry or stern expression and two viewports serving as "eyes"." are clearly defined in the image attached to this wiki page.While the rest of the limbs that are joints of hard light are not entirely visible, it can be assumed they are fully transparent in the lighting in the background as a plausable theosis that this is a War Sphinx.The reasons that this is NOT a Seeker are clearly explained in the Seeker page:Seekers were longer and more streamlined than War Sphinxes, and were equipped with more powerful movement and defensive capabilities. They lacked the "face" that adorned the command cabin of a War Sphinx.Note, it says "They lacked the "face" that adorned the command cabin of a War Sphinx.", the above image of the unidentified Forerunner construct CLEARLY shows a "face" in the form of a what is assumed to be the command cabin of a War Sphinx.I would also like to mention that although neither reference "tenticles", they could very well form the "tail" of the War Sphinx.Refs:War Sphinx - http://halo.wikia.com/wiki/War_SphinxSeeker - http://halo.wikia.com/wiki/Seeker
Sigh... Sorry it failed to format properly...