I'm creating a revised beast master ranger and decided to get a pet panther. Since my pet gains ASIs whenever I do and we're using the optional feats rule, then can my pet get feats instead of ASIs too?
When asked if it was the intent to allow the beast companion to take feat instead of ASI, or if he personally would allow it, Jeremy Crawford answered:
No on both counts.
No, the animal companion isn't able to choose feats instead of an ability score improvement.
The Unearthed Arcana: Revised Ranger states:
Whenever you gain the Ability Score Improvement class feature, your companion’s abilities also improve. Your companion can increase one ability score of your choice by 2, or it can increase two ability scores of your choice by 1. As normal, your companion can’t increase an ability score above 20 using this feature unless its description specifies otherwise.
Your animal companion isn't gaining the Ability Score Improvement class feature. Their ability scores only increases alongside yours if you gain that feature1.
Since gaining a feat requires you to forgo taking that specific class feature (PHB, pg.165), not just giving up increasing your ability scores, your animal companion can't forgo what they don't have.
1 Side note that if you don't take the ASI and instead choose to gain a feat, your animal companion's ability scores don't increase either.