The sentence is missing a pronoun, though! Here's why:
* Pronouns replace nouns. We need a pronoun to stand in for the triceratops, which is the noun performing the action.
* "His" is a possessive pronoun. It shows ownership, but not the subject performing the action.
Here's a corrected sentence with a pronoun:
"It filled its belly with the cold water from the stream."
It replaces "the triceratops" and is the subject of the sentence.