Sentinel Prime basically had the same robot mode before he scanned the Rosenbauer Panther fire truck, so on Cybertron, he had to have an ALT mode, right? He wasn't a protoform, so again, he transforms into something. Check this out, a photo I found of a possible Cybertronian ALT mode. That's just fan art, so it's not official, but I like the look, it says "Sentinel" IMO. But he could be something else, maybe a jet or something? What do you think?