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

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

2 answers

#2 is correct. No comma is needed because "that" introduces an essential clause.
http://grammar.ccc.commnet.edu/grammar/clauses.htm#independent

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