Is it rare to have tendonitis in outside of foot?

Not really rare: Peroneal tendons are on the outer border of foot and help extend and lift your foot outwards so can be brought on by shoe wear changes, training changes, foot conditions.

Rare: Yes, if you are indeed dealing with tendinitis, this would be a rare finding on the outside of the foot. One of your two peroneal tendons are involved. Have your local area podiatrists assess your range of motion in both the rear foot and forefoot and even ankle. Take x-rays for calcification in either these two tendons. Conservative is first.