Needs review
Source: Races of Stone

Description: You are particularly adept at maneuvering mounts through tight spaces and underground passages.

Prerequisites: Mounted Combat, Tunnel Fighting

You and your mount do not take penalties on attack rolls or to Armor Class when squeezing into or through tight spaces. You can fight in any space large enough for the mount to squeeze through. Your having this feat does not allow a mount to squeeze through a space it would not normally be able to pass through.