The question contains pairs of words that are related to each other in a certain way. Three of the following four word pairs are alike as these have the same relationship and thus form a group. Which word pair is the one that DOES NOT belong to that group?
(The words must be considered as meaningful English words and must not be related to each other based on the number of letters/number of consonants/vowels in the word.)

The correct answer is Hexagon - 8 sides.

Let us analyze the relationship between the words in each of the given pairs:
1. Pentagon - 5 sides: A pentagon is a geometric polygon that has exactly 5 sides. This pair correctly represents a shape and its number of sides.
2. Triangle - 3 sides: A triangle is a geometric polygon that has exactly 3 sides. This pair correctly represents a shape and its number of sides.
3. Square - 4 sides: A square is a geometric polygon that has exactly 4 sides. This pair correctly represents a shape and its number of sides.
4. Hexagon - 8 sides: A hexagon is a geometric polygon that has exactly 6 sides, whereas an octagon has 8 sides. Thus, this pair does not correctly represent the number of sides of the shape.

Three of the four pairs (Pentagon, Triangle, and Square) follow the relationship of "Polygon - Correct number of sides". The pair "Hexagon - 8 sides" does not follow this relationship, making it the odd one out.