While the general assumption is that this was Doomsday preparing to fight the Trinity in the Batman vs Superman trailer, I am not convinced. Check out the trailer to see what I mean...
It Doesn't Really Look Like Doomsday
Hollywood is known to take liberties with character design, but this is very different from any iteration of Doomsday we've seen in the comics. It is also not original at all, bearing a striking resemblance to both Abomination from The Incredible Hulk and the cave trolls from Lord of the Rings. Why ruin Doomsday's distinctive look? Further, if it is Doomsday, they've completely thrown away his entire backstory and history. It would seem a really poor choice to create so much hype, only to rip off your competitor and bastardize one of your most powerful and popular villains.
Also, Doomsday never had the same heat vision as Superman. Do they really want to follow in the footsteps of the classic Superman 4, where they just gave Superman a bunch of random powers, like rebuilding brick walls with his mind?
It Is WAY Too Early In The DCCU To bring In Doomsday
Batman vs Superman is only the second movie in the DCEU, and is already stuffed to the brim with characters. They've got Superman, Batman, Wonder Woman, Lex Luther, and anticipated cameos from Cyborg, Aquaman, and The Flash. I understand the need to create a threat that is physically capable of taking any one of the Trinity out, but there are plenty of other options without wasting one of the most powerful and popular villains in the Superman canon. Remember, in the comics, Doomsday took out the ENTIRE Justice League, and killed Superman.
If Not Doomsday, Then Who/What?
Well, we know that Lex Luther is in it, and he's typically villain enough. That said, they likely want a foe that is so powerful the Justice League has no choice but to form. We also know that Lex is playing Mad Scientist, and is (per usual) hell bent on taking Superman out. Given what we know, I see a few potential options, discussed below, starting with the least likely:
Solomon Grundy was murdered and resurrected as a largely mindless, violent, erratic zombie. He certainly fits the bill, in terms of strength, and his looks are no more different from the monster in the trailer than Doomsday is. Still, this is unlikely. Grundy lacks name recognition, and it is unlikely he would give the Trinity that big of a fight. Also, no heat rays.
Resurrection and Genetic Modification of Zod
We know that Zod is powerful enough to stand toe-to-toe with Superman. We also know that Lex Luther has Kryptonite in his possession. He could have found a way to fuse some of that with a resurrected Zod, to give him an edge. This would provide a nemesis powerful enough to take on Superman, but not Superman with Wonder Woman and Batman. If they wanted an opponent that physically fits the bill, while not using any established villains, this would be the way to go. Especially considering Lex Luther has been touted as the villain of the movie. However, this is very close to the backstory of a more established Superman villain...
In the Pre-Crisis storyline, Bizarro was originally created on Krypton as a clone of... General Zod. While they were essentially his foot soldiers (no yellow sun on Krypton to give them powers), this is quite the coincidence. The Bizarro on earth has a similar backstory, being created as a clone of Superman that did not turn out too well. This backstory is a lot more similar to what the trailer showed. Bizarro is also not nearly as well known or popular as Doomsday, meaning fans aren't going to be nearly as protective.
It Is All A Smoke Screen
It does seem odd for a major blockbuster to essentially give away the entire plot of the film in the trailer. If we are to believe this is essentially it, then it's a pretty boring story. Batman and Superman don't like each other, Lex Luther uses this to push his agenda hoping Batman can take the alien out, Batman and Supes fight, realize they are both on the same side, out comes Doomsday, Wonder Woman shows up, and they all go after the bad guy.
I am not buying it. They can't be THAT stupid, to put so much time, money, and hype in to what would essentially be a cookie-cutter story, only to give it all away 4 months before the movie drops in a silly trailer. It just doesn't add up. I believe there has to be a MAJOR twist or two that is being held back, and it is likely that part of that is that this cave troll is not Doomsday.