No Guard ensures that all attacks used by, and targeted at, the ability-bearer hit without fail.
Headbutt is Normal type, but it hit your Honedge because it had the ability No Guard.
EDIT:
As your comment below you have said that Scratch and Tackle do not have any effect on the Honedge, so, either I am misinterpreting how No Guard works, or the Zigzagoon used Scratch and Tackle and then used Odor Sleuth.
>Resets opponent's Evasiveness, Normal-type and Fighting-type attacks can now hit Ghosts, and Ghost-type attacks hit Normal.
It must have used Scratch and Tackle then used Odor Sleuth.
Hope I helped!
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