The Party Animal trait is a social trait for Sims who enjoy parties and like making sure everyone else knows it. The Party Animal trait is available to Teen Sims and older. It is part of High School Years, so you need the expansion pack to select it in Create a Sim.
Party Animal Sims have several party-related social interactions. They can perform party tricks, rave about parties, remember past parties, perform the Woo interaction, and do Vibe Checks on other Sims.
Party Animals also get the In My Element moodlet while attending or hosting a party. This is a +3 Happy moodlet that lasts for the duration of the party and continues for another four in-game hours after the party ends.
Sims with this trait will get Tense if they attend parties once in a blue moon.
The trait conflicts with Gloomy, Grouch, Loner, and Socially Awkward. These traits cannot normally be selected together with Party Animal in Create a Sim.
The Party Animal trait is separate from the Party Animal aspiration.
The Party Animal aspiration is a Popularity aspiration focused on throwing and attending social events. Completing it gives the Perfect Host reward trait, which gives a boost to the score of social events the Sim throws.
So, choosing the Party Animal trait does not mean your Sim has completed or selected the Party Animal aspiration. They are two separate features.