Snakes are dangerous if they bite.
They is a pronoun. Snakes is an antecedent. The antecedent is the word that the pronoun is about.
A pronoun must always agree with its antecedent:
Masculine - James cleaned his car.
Feminine - Susan took her dog for a walk.
Neutral - The clouds have changed their direction.
Singular - The student carried his book.
Plural - The students carried their books.
Now choose the pronoun which agrees with the antecedent.