They usually aren't very large, maybe the size of a large dog, but larger ones could be created. The number of legs can vary, but they usually should have at least six legs, but no more than ten legs. Some have the ability to shoot webs out of their mouths. Most flesh spiders are poisonous, with a mouth filled with sharp fangs.
The flesh spider isn't that powerful, and the reason for such a creation is unclear. They can't take much damage, and when and if they do take damage, something unusual happens.
When a flesh spider is encountered it doesn't always attack. It can taunt whoever it encounters, and sometimes it will say something about a character it shouldn't know about. It may also spout gibberish, wail, or mutter. Use your imagination.
A flesh spider is immune to sleep and charm spells. It cannot be controlled with magic.
Alchemists and magic users will pay a very good sum of gold for an alive and intact flesh spider. It's various glands and ichor are useful in many alchemical preparations, and as spell components. However, as many alchemists have discovered, dissecting a flesh spider is a very tricky procedure.
Once a flesh spider takes over half it's hit points, roll on the following table, or make your own table; to determine what happens.
I left out game specific stats, so the GM/DM/Storyteller can customise them to suit their campaign. I hope it's creepy, weird, and chaotic enough.1. Sticky fluid starts spewing out the wounds, and could trap characters to durable surfaces like stone floors, etc. Clothing is ruined, and it's very difficult to clean off of armor, and weapons. It's very stinky.
2. The wounds begin oozing a very strong acid.
3. The flesh spider turns inside out, and grows into a much larger, fleshy blob with tentacles.
4. Thousands of small spiders start pouring out of the wounds, and attack.
5. A red gas starts spewing out of the wounds until the spider is dead. All the red gas does is make who ever comes in contact with it itchy. If the gas is inhaled the character develops a cough for a few days.
6. The flesh spider explodes and a minor demon appears.