Aliasing six_arms → 6_arms
Link to alias

Reason:

Cleaning up/consolidating tags to accepted format,
Here, have a bunch more:
one_arm aliased to 1_arm (low post count, may not be worth it)
two_arms aliased to 2_arms (low post count, maybe irrelevant as that is the default number on most humanoids, may not be worth it)
eight_arms aliased to 8_arms (low post count, may not be worth it)
nine_arms aliased to 9_arms (post count 1, may not be worth it)
ten_arms aliased to 10_arms (post count 1, may not be worth it)
twelve_arms aliased to 12_arms (post count 2, may not be worth it)

four_legs aliased to 4_legs or maybe both aliased to quadruped
five_legs aliased to 5_legs (low post count, may not be worth it)
six_legs aliased to 6_legs
eight_legs aliased to 8_legs (low post count, may not be worth it)
ten_legs aliased to 10_legs (low post count, may not be worth it)
twelve_legs aliased to 12_legs (post count 1, may not be worth it)

The intervening numbers could also be aliased, but most of them have no posts or maybe just one between the two variants of tag.