There are a variety of scary and humorous monsters randomly populating each level that will try to stop your character from collecting items and reaching the next level. As you advance in levels, enemies from further down this list get more common.

There are currently eight enemies in the game:

Photo Name Characteristics
IMG 3915
Cactus Turtle Moves in random directions slowly. Bears and pogosaurs are defeated when they run into it.
Yeti Throws snowballs at the player from a distance. The snowball freezes players with their current momentum, which can carry them off ledges or into spikes or enemies. The Yeti runs away if the player gets too close. Unlike all other enemies., the player can safely touch Yetis and push them around, even off of cliffs.
IMG 3920
Brown Bear Will remain stationary and asleep until the player comes within range, and then marches directly towards the player at a moderate pace. If the player gets far enough away, it will go back to sleep. The player can safely touch the bear from behind, which is generally only possible when it is asleep or just waking up. 
IMG 3924
Pogosaur Waits in place until the player comes within range, then leaps directly towards the player on its pogo stick at a moderate pace. Unlike the brown bear, it will continue pursuit no matter where the player is in the level. The player can safely pass under it during the peak of its jump.
IMG 3932
Bull Waits in place until the player comes within range, then it gets alerted a couple seconds later charges very fast at the player's position. It can only slightly bend its trajectory after the charge begins, so the player can lure it in a direction and then sidestep it. Defeats brown bears, pogosaurs, yetis, and cactus turtles on contact.
Black Bear
Black Bear Remains stationary until alerted like the Brown Bear but never stops pursuit and it's much faster. Will defeat all enemies except for cactus turtles.
Cloud Spider Invincible, hovering above the level. Periodically drops large poison bombs over large hexagonal areas near the player. The area lights up beforehand to give the player time to react. There are two types of Cloud Spider: ones already present in some levels, and ones that will appear after the player steals 10 gold from a certain pedestal.
IMG 3934
Ghost Invincible. Fast mobility. Only appears after player takes any item from a grave and then never stops pursuit. Will defeat any enemy it passes across.