Regardless what size your dragon is you will eventually encounter an enemy that will cause you problems. At the start of the game target flocks of birds to get plenty of point totals so you can level up your dragon quickly. If your dragon is short on life you need to go after larger creatures such as sheep and people because they give you large amounts of life. The following is a list of common things your dragon needs to avoid.
Mines
These are randomly placed big, yellow spiky things which will explode if you crash into them. They will injure your dragon by depleting it of about 20 percent to a quarter of its health. If you are running low on health you must avoid them at all costs.
Red Bugs
Avoid these insects as they will take a bite out of your health given the chance. The purple bugs are fine to eat, it's just the red ones you need to stay away from.
Glowing Green Creatures
Avoid any creature that glows green as these are poisonous and will poison your dragon.
Farmers and Soldiers
Although eating humans can increse your health they can also be dangerous to your dragon. Human farmers are armed with bows and arrows which can would you if you get shot by one and soldiers who have spiked bats which will deplete you of a significant amount of health if they hit you with it.
Witches
In the later stages of the game you will encounter witches which you can eat without any problems but if they manage to blast you with their speslls your dragon will be cursed which means it will progressively lose health for a few seconds.