This was something I thought about back when we first heard the name of Cogman... as in, it sounds soooo lame. But then, put yourself in the shoes of someone from the middle ages, before there was science - what would you call an alien made of machinery and cogs?
To be honest, it would have probably made more sense to be called Metal Man, as that would have been the only component humans at that time would have been familiar with (cogs would have been more from the industrial era anyway).

Unfortunately, the flaw of all this is saying that the humans gave him his name... when obviously he should have had his own name before he came to earth. Or if he was built here, the Cybertronians who built him would have given him a more Cybertronian name.
In other words, when and how did he come to Earth, and why would the humans name him... unless it was like a nickname, and he adopted it as his own name because he liked it.
This is the sort of unexplained element that an IDW comic would have explained, if they had kept doing movie comics.