Doggy Diagnosis, red blemishes/rash on belly

[JHoNNy1OoO]

I've got two dogs, a medium sized 10 year old mix and an 8 year old chihuahua. Two days ago I went to give both of them a bath and noticed the chihuahua had a couple of red marks on his belly/in between his hind legs. Didn't really think much of it and continued with the bath. Yesterday I went to look and the red marks were disappearing and figured it was just a mild irritation from brushing up against something outside. Looked today and it seemed to flare up again this time a bit more aggressive.

Nothing has changed in his environment or his food and he's crapping just fine. Throughout the whole time he hasn't been scratching or licking them at all. He lets me put my hand on them and acts like they aren't even there. The area seems to feel a little warmer than normal but nothing extreme. The chihuahua is the only one with the red marks, my other dog shows zero symptoms of anything and they're both together all the time.

I'm taking the little guy to the vet tomorrow morning, but was hoping to go in there with some knowledge beforehand. Anyone experience anything like this with their dog? Any help is greatly appreciated!

Pics!
Chest area - http://i.imgur.com/u074t.jpg
Manhood area - http://i.imgur.com/xCiri.jpg

 

[MikeMike]

Fleas/Tics/etc?

 

[Jaepheth]

It's Fin Fungus; he'll be floating upside down by morning.

 

[Azraele]

Definitely a good idea to see the vet. Could be anything from fleas to a food allergy.

 

[JHoNNy1OoO]

Fleas/Tics/etc?

None that I have seen or found.

 

[GoodRevrnd]

Wow that looks pretty severe. My dogs have seasonal allergies that cause skin irritation and itching that will eventually develop into some of the less severe blemishes pictured but yours seem too widespread and randomly blotchy and severe for that to be the case (plus they showed up over a few days?). They get treated with a topical steroid spray that mostly keeps their itching and rashes in check.