Rabbit has black rock like lump on back

Question
Hi, I have a 5yr.old dwarf lop male rabbit. He had a black rock like lump on his back, I took him to a vet. who tested it for 2 types of insect, the results were negative.  Now the rock like bump is back & there looks like there might be some white on it. There also seems to be another same lump a little above it.   What could it possibly be?  

Answer
Dear Marie,

Without seeing a picture, it's very hard for me to guess what this could be.  How big is it?  Does it move freely with the skin, or is it firmly attached to underlying tissue?  You say it's "rock like".  Does that mean it's very hard, or does it give a little bit when you touch it.

If you could somehow take a picture and send it to me via email:  dana@miami.edu  I might be better able to help.

I'm guessing that if the vet checked for "insects" there was a suspicion that this could be a cyst caused by a botfly. The cyst is usually not black, but if it is an old cyst (i.e., the botfly larva already has dropped out and metamorphosed into an adult fly), with some dried pus inside, this *might* be what you're seeing.  Again, I'm only guessing, since I can't see it.

I can't guess what other type of insect the vet might have looked for, but if you can find out, I might be able to tell what this lump looks like.  Or if you can give me a *very* detailed description, and whether or not the bunny seems bothered by it, this also could help.

Dana