Of course not. Mods and Texture Packs are also used to get cool stuff. You could even create your own; there are plenty of tutorials out there, you know!
For Horses, as long as you have Minecraft 1.6 or higher installed, you can find horses randomly generate without a seed. For Ender Dragons, you need to travel to The End with the Ender Portal.
To get to the Ender Portal, you will need at least 13 Ender Eyes; you can get Ender Eyes by killing an Enderman. Aim to a point in the sky with the Ender Eyes selected, and right click. Follow the Ender Eye until it lands. Dig down where the Ender Eye landed and you should find a stronghold. Place an Ender Eye into each one of the holes of the Ender Portal (there should be 12). Then, jump in!