A spider eater is about 10 feet long and 4 feet high, and has a wingspan of about 20 feet. It weighs about 4,000 pounds.

A spider eater attacks with its venomous sting and powerful mandibles. Its usual tactic is to deliver a sting, then back off, hovering out of reach until the venom takes effect.
Implant (Ex): Female spider eaters lay their eggs inside paralyzed creatures of Large or larger size. The young emerge about six weeks later, literally devouring the host from inside.
Poison (Ex): Injury, Fortitude DC 17, initial damage none, secondary damage paralysis for 1d8+5 weeks. The save DC is Consitution-based.
Freedom of Movement (Su): Spider eaters have a continuous freedom of movement ability as the spell (caster level 12th). When the spider eater serves as a mount, this effect does not extend to its rider.
Skills: Spider eaters have a +4 racial bonus on Listen and Spot checks.

A spider eater requires training before it can bear a rider in combat.
Training a spider eater requires six weeks of work and a DC 25 Handle Animal check. Riding a spider eater requires an exotic saddle. A spider eater can fight while carrying a rider, but the rider cannot also attack unless he or she succeeds on a Ride check.
Spider eater eggs are worth 2,000 gp apiece on the open market, while young are worth 3,000 gp each. Professional trainers charge 3,000 gp to rear or train a spider eater.
Carrying Capacity: A light load for a spider eater is up to 306 pounds; a medium load, 307?612 pounds; and a heavy load, 613?920 pounds.

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System Reference Document -> List of Creatures (SRD)