My first guess was porcupine 🤥, but when I counted the letters it didn't sum upto 11
So I had to use the clues given to find the word
It was so hard to find, but I came up with my guess, I think the answer is "pycnogonida"
Which using the clue
1: it is 11 letters
2: you find it in nature
3:it is brown and greyish in color
4: it is 60-90 cm long
5: it can hurt

pycnogonida