What could the small extension next to one of my testicles be? the size hasn't changed since i noticed it as a kid and doesn't feel any different.

Epididymis. : To be reassuring, what you are describing sounds like an epididymal cyst, a very common, benign, and non-cancerous condition. Masses outside of the testicle itself are rarely malignant. However, i recommend you get an exam to make sure it is not something that may warrant a testicular ultrasound. I applaud your use of testicular self examinations. Continue them to see if you notice any changes.