The Endless Plains area is disarmingly close (southeast of) Oleg’s Trading Post - just north of Fangberry Cave. That said, this is not a location for beginner characters, as the description points out a “manticore [has] claimed the plains for its hunting grounds.”
Sure enough, if you explore the area by heading east a bit a Manticore will swoop down from the sky and land. Apparently adventurers are on the menu. A Manticore is not a monster for low level parties… or at least, not for parties new to the Stolen Lands. When you’re 4th-level and have a full party, you’ll have a better chance, so if you’re not up to snuff, don’t sweat it - complete other quests, explore other areas, and come back here later.
This particular Manticore boasts an Armor Class of 18, which is far from insurmountable. Its 60~ Hit Points will allow it to weather some damage, however. Where it’s really dangerous is on offense, having multiple attacks and a fairly high Attack bonus. By the time we challenged the beast at 4th level, our Monk/Magus protagonist’s Armor Class could exceed 30 when buffed, making this a rather easy fight, but any of our animal companions (similarly buffed), would have fared just as well. Use spells like Mage Armor, Barkskin, Shield, Shield of Faith, Blur and Mirror Image to get your defenses up to snuff, lead with that character to draw the Manticore’s attention, then try to flank it to bring it down quicker. Not a terribly complex fight, just a stat-check to exclude the weakest of parties.
Once it’s dead you can find several corpses strewn throughout the small area. One near where the Manticore landed (southeastern corner of the map) will yield a Belt of Incredible Dexterity +2, a second to the north of the first hides a Scroll of Touch of Gracelessness while a third near where you entered (western edge of the map) can be looted for a Scimitar +1, and a Wand of Grease, among other, less interesting prizes.
Not a bad haul at all for an early party willing to risk life and limb against vicious beasts.