What does the poet mean by " Because the dentist may be a bear, or as the Romans used to say, only they were referring to a feminine bear when they said it, an ursa,".Its from "This is going to hurt just a little bit" by Ogden Nash.
This whole line is just comparing the dentist's action to that of a bear. He digresses a bit by giving the latin name for bear "ursa" which is what the Romans would call the animal.
Why does he write like this? I think it is to show in the character of a man who is comically frightened of going to the dentist. By writing in this rambling way he communicates the confusion in his mind.