Is It The Bit?
If a horse is sticking out his tongue only when bridled, it’s most likely because the bit is bothering his mouth, explains top trainer/clinician Julie Goodnight. http://trailridermag.com/article/bit-14763
Resources for bit pressure
Bitting System
Using The Goodnight Bitting System The self-correcting bitting system I use is often called an elbow pull and it is a long harness leather cord with snaps on each end. To use the bitting tool,