Arachno
Arachnos aren't too hard, but they will definitely eat the average newbie. They deal out a truckload of slashing and bleeding, unless you have Superclot. Dodging is a good defense against their attacks; for this reason (along with the fact that they can't take hits very well), you will often find people fighting them naked. Their contract payout is good: $500 per arachno, with a maximum contract value of $15,000.
You can find arachnos in the subsewers, underneath the Bug Queen. You need to kill her to get there, but she isn't very strong.
When you start hunting them, keep in mind that they're social and occasionally aggressive. Webbers and sniffers tend to be more aggressive than drones and workers, and are more likely to assist as well.
Their claws are considered a PC2 weapon, so fist users and players using lighter weapons can expect to see lots of parries.
The biggest threat in the subsewers tend to be the Arachno Queens, who aggro extremely quickly, and hit like a train. They also seem a little more accurate than normal arachnos, so the best defense against them is parrying or stunning them.