Were they orginally made from real dogs?
The term "dog" has been used as a synonym for sausage since 1884 and accusations that sausage makers used dog meat to at least 1845.
According to a myth, the use of the complete phrase "hot dog" in reference to sausage was coined by a newspaper cartoonist in 1900, but there were several earlier references, but no specifics on the origination of the phrase.
Thesaurus.com has an article on the name. Their explanation is that the long sausages got compared to dachshunds. With time, people started calling them "dogs" instead of "dachshunds".
They don't list a source for the information, but I hope that, being language experts, they have fact-checked it.