Rejected names include 'bitey shrieker', 'unhappy yelper' and 'angery caterwauler'.
Anyways, just another magical beast. 7 RHD and medium size aren't awesome, but workable. The non-constitution stats aren't that impressive (18 strength, 17 dexterity, 7 intelligence, 14 wisdom, 16 charisma), but constitution itself is a whopping 27.
Main natural weapon is a 1d8 bite, backed up by 2 1d6 claws. About expected for a medium feline: so not great.
The special abilities are partially expected (Trip, Darkvision, Scent), but also include a few surprises. Minor Displacement gives sight-based attacks a 20% chance to miss, making it a useful passive boost. Yowl of Fear is nice, but ultimately pretty useless as it takes up an action and only causes a mild condition (Shaken). Fast Healing is rarely seen on creatures like this, but not unwelcome. Finally, there's immunity to Fear, Negative Energy, and Energy Drain, which is nice to have if nothing else.
Is the feral yowler powerful? No: for 7 RHD you could have a magical beast that's bigger, stronger, and capable of flight. Even the regular displacer beast, with only 6 RHD (and +0 LA), is more interesting on account of eating up less levels, having superior displacement, and being larger. That said, its immunities and fast healing give the yowler a decent niche, I'd argue.
While I think a case could be made for +0, I'm going to assign -0 here and see what people think. Do discuss!