Adjectives have three forms. A base form, a t-form and an e-form. If the word the adjective modifies is a common gender noun the adjective is used in it's base form. If the noun is a neuter gender the adjective is used in it's t-form, which adds a t to the adjective. And in plural it takes the e-form which adds an e to the adjective. To top this off there's also irregulars which deviate from this in different ways.
So, "træt" is an irregular in two ways, a: there's no difference between the base and the t-form. So, no matter what gender noun, in singular it will only take the base form which is "træt". And b: in e-form the last consonant doubles before the e is added.
"Drengen er træt."
"Dyret er træt."
"Pigerne er trætte."