There are the summons, many of which make appearances all over the series. There is materia in more than one. There are lots of giant swords.
There are also some enemies that have turned up in nearly every game since the inception of the series.
One of those enemies is Behemoth, and a handy youtube video from ♥RinoaMoogleGames has compiled the evolution of this enemy, across time and console generations.
Here are the first 5 iterations of Behemoth, starting with FFII in 1988 and ending with FFVI (my personal favorite) in 1994. You can see the way the art evolves and becomes more distinct with every new game.
I separated the games into the first five and last 9 because at FFVII, the medium changed, the graphics improved, and it moved from 2D pixels into 3D polygons.
Here's after the series went 3D, starting with FF7 in 1997.
The Behemoth has taken on size and shape in these iterations, and only gets more and more defined as time goes on, all the way up to the XV demo footage released last year.
I don't know why, but seeing the way this thing has evolved and taken on shape and form over time gets me excited for XV. I haven't really played a Final Fantasy game in a while, except for a copy of the GBA VI on an emulator.
I'm getting excited about XV for all the goodness it looks like it contains, and the beautiful rendering of what we've seen so far. It feels nice to get excited about Final Fantasy again.
If you want to check out he original video, here it is. The channel also has the evolutions of Shiva and Malboro, if you're interested.