Using the seven note natural minor scale as a starting point, we remove two notes to create the five note of the minor pentatonic.
Again, the notes removed are those which are a half step from the chord tones of the 'one' chord in the given key.
For example, taking the A minor scale, an Am chord was created from the 1st, 3rd and 5th notes of the scale:
A B C D E F G A
The two notes a half step from the chord tones (namely B and F) are removed to avoid any potential for clashing, dissonant notes in a melody over the Am chord.
The notes of the A minor pentatonic are therefore:
A C D E G
If we identify those notes in intervals by their scale degree in relation to the tonic (A) we will have :
1 ♭3 4 5 ♭7
An E minor pentatonic scale would therefore be:
E G A B D
A B minor pentatonic scale would be:
B D E F♯ A
The formula for the minor pentatonic horizontally is therefore:
Minor 3rd-Whole-Whole-Minor 3rd-Whole