My Rocky had a really BIG swollen penis one time.... I brought him to the vet because "I didn't know what to do with it"!!
His skin around the penis is loose enough to just slide it back over it.... the vet showed me what to do... you could run it under water or use baby oil to get in back in .... in case you are worried about it getting dirty while it's hanging out all swollen ...
Now I know what to do.... so it's no "BIG DEAL" anymore!!
