Accessory soleus

Most common accessory muscle in ankle
Most common soft tissue mass seen in Kager's fat pad

Clinical features:
pain or mass in posteromedial ankle
May cause compressive neuropathy of posterior tibial nerve

MRI:
isointense to surrounding muscle and courses anteriorly relative to tendo achillis

References:
Ly JQ et al. Anatomy of and Abnormalities Associated with Kager's Fat Pad . AJR 2004; 182:147-154