My dog has a pea-sized lump on his testicle. Should I worry?

Check the other testicle in approximately the same location where you found the lump on the first testicle. If you can find another one like it, then you are feeling the head of the so-called epididymis , which is the sperm-holding part of the testicle and belongs there. If the lump you noticed is a third lump (ie there is none like it in the other testicle and you can also find two identical lumps in the testicles) then this needs to be examined by the vet as it may be a tumor, cyst or little abscess.