This ad states that Hydrox is "the original cream-filled chocolate cookie" and they did indeed come first - they were first sold in 1908 and the Oreo came along in 1912; the Oreo, made by Nabisco in New York City (the street the factory was on, 9th Avenue between 15th and 16th, is now called Oreo Way).
No one seems to know why it is called an Oreo. It might have something to do with "or," the French word for gold. And it might not. As for Hydrox, the name was a portmanteau word made from hydrogen and oxygen. It was supposed to stand for a modern, scientific goodness in your sandwich cookie. The Hydrox IS science.
Oreo on the other hand was unscientifically golden. It became the best selling cookie in the US and it has been that way for the Oreo ever since. Poor Hydrox. I've never had one but I gather that they were a bit less sweet, and less crumbly when dunked, than Oreos. A lot of people loved them, according to this Wall Street Journal story. If you were a Hydrox fan, you were (I gather from the WSJ) a little edgier, a little more "in the know." It was the hipster of cookies. Oreo was for the hoi polloi.
But alas for Hydrox. Between 1999 and 2003, Keebler (who bought the Sunshine Biscuit Co. in 1996) made
If you are in the US, that is. I'm in Canada so I probably won't be looking. There is a Canadian version of the Oreo/Hydrox though. It has a surprisingly bossy name (ironic, because Canadians are are so agreeable and polite, you know): Eat the Middle First. This is the cookie that orders you to approach it in a very specific way. This cookie does not want to be dunked headfirst into a glass of milk. It wants to be analyzed and deconstructed like a poem. What do you think it is, a doughnut?
And the convenience-friendly, ready-to-eat, instant-coffee version of all this is over on Tumblr.