Swollen lump on dogs anus.(pics inside) What is it?

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Since about 4 monthso on the left side of my dog's anus(sorry i dont really know the specific terms) there is a lump or it is swollen. He can poop normally with no problem , and he acts healthy as usual. He is about 8 years old . I took some pictures , can you people help me know what is this?

Note : its pictures of my dog's ass with a lump next to it.

http://imgur.com/a/1Sxg5#0