I call it their peeper. Once before Sugar was neutered I was picking him up from the groomer. He had a full on erection!

Now on a three lb. dog, that takes up the whole underside of his body. The groomer was bringing him to me and telling him to put that thing away. The other groomer said, does he have his little crayon out? My groomer turned Sugar so she could see and she said OMG! That's not a crayon that's a magic marker! Oh Lord! The joys of little boys!

Guess he likes the groomer though.
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226