As far as I can tell, scarlet spiders don’t really exist. (Hypsosinga rubens probably comes the closest in our world, but it’s an orb-weaver, not a hunting spider.) But they're great fantasy spiders, in the sense that they’re instantly recognizable as dangerous and seem like they’ve come straight from the pages of a fantasy novel. (They’re exactly the kind of spider the crafty vizier or power-hungry duke would introduce into the bedchamber of a rival.) With their iconic appearance and a bite that saps its victims’ strength, it's no wonder they are prized by spellcasters of a predatory bent.
Meanwhile, goliath birdeaters in our world are known for hunting birds (obviously), though frogs and invertebrates are more commonly on the menu. Whereas the goliath spider in Pathfinder, being of Colossal size, is big enough to hunt dragons and giant scorpions…but adventurers will do in a pinch.
A hunting party of phanatons (treat as ratfolk with the Gliding Wings (3 RP) movement racial trait from the Advanced Race Guide) invite adventurers to share their rather ingenious camp inside the shed husk of a male goliath spider. Ingenious, that is, until a female goliath spider comes crashing through the canopy, intent of devouring the corpse.
Every Marday a court magister sends his scarlet spider familiar to bite the dauphin. The weekly attacks have left the heir in fragile health for most of his life. As the magister is also the one who treats the boy, his betrayal has so far gone unremarked. But when adventurers come to visit, bringing with them a cleric whose healing power outstrips the magister’s own, the man grows convinced that he will be found out. Convinced…and reckless in his desperation…
Adventurers are passing an old cemetery in the scrublands when the earth erupts and a goliath spider lunges forward. What they had taken to be a cemetery is actually an artificial thing of dirt and stone slabs webbed together into a trapdoor for the spider’s lair. While the goliath trapdoor spire is mostly interested in their mounts, the adventurers will have to survive falling debris and their own stampeding steeds.
—Ultimate Magic 117–120 & Pathfinder Bestiary 4 252
I’m not a big minis guy, but Games Workshop’s Arachnarok Spider would make a pretty sweet (if slightly small) goliath spider.