I'm sorry to hear that your dog is having this issue. Vomitting blood clots and later vomitting dark liquid (may be digested blood) can indicate several things. This may be from ulcers, GI bleed, or foriegn object ingested (such as plastic if it was along with the hotdogs). You can read more here: http://www.petplace.com/dogs/hematemesis-vomiting-blood-in-dogs/page1.aspx Further, dogs can have problems with onions so consider checking the ingredients of the hotdogs as well. It would be advisable to have him into your vet as he should be seen face to face for this. After his visit, you'll likely want to keep him on a bland diet for a few days (boiled chicken and rice or a prescription food like I/D). I hope he's doing better with this soon.
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