Based on their size and weight:
* "As big as a hippo": This is a straightforward idiom meaning very large.
* "To be weighed down like a hippo in mud": Meaning to be burdened or slowed down by something heavy or difficult.
* "To move with the grace of a hippo": This is a humorous idiom meaning to move clumsily or awkwardly.
Based on their temperament:
* "To have a hippo's temper": Meaning to be easily angered or aggressive.
* "To be as stubborn as a hippo": Meaning to be unwilling to change your mind or give in.
Based on their habitat:
* "To be in your element like a hippo in the water": Meaning to be comfortable and happy in a particular environment.
Humorous idioms:
* "To be as out of place as a hippo in a ballet class": Meaning to be completely out of place in a particular situation.
Remember, these are all made-up idioms, but they capture some of the characteristics of hippos and could be used in a humorous or creative context.