Question

1. An insect is a small creature, that has six legs.

2. An insect is a small creature that has six legs.
=================
Which one is grammatical?
Is #1 wrong in terms of grammar?
Can we put 'comma' before 'that' as in Sentence 1?

Answer 1

#2 is correct. No comma is needed because "that" introduces an essential clause.

Answer 2

http://grammar.ccc.commnet.edu/grammar/clauses.htm#independent

Essential clauses and Restrictive clauses are two terms for the same thing!