huge lump

i know it is not melanoma but my grey warmblood gelding, got a huge lump under his right arm near the chest,,  he could not walk on it,, i gave him so banamine and brought him inside for some  tlc,  i rubbed it and cold hosed it and the next day it seemed to have developed a drain,, i put hot compress on it and i rubbed it and it opened more and i put fura ointment on it to keep it open . it has drained and i'm keeping it clean and open  oh it is called a hematoma  do not give bute as it will cause it to bleed only banamine in the mouth