"'There is'/'There are' several ..." in this sentence

Solution 1:

You are correct in your assumption that the sentence is grammatically incorrect by accepted standards of modern English. If you reduce the sentence to its basic structure (noun, linking verb, predicate nominative), there remains the complete sentence, "there is horses." Obviously this does not comprise correct subject/verb agreement. I see how the word several could be confusing, but in this case it is an adjective describing how many horses there are.

Solution 2:

I agree with dusktreader that this is not common in contemporary English. But I think the phrase "there is" is used statively here, many other Indo-European languages have certain constructs where a stative is used, or often implied. It means something like the following;

There is (a situation in which) several draught-horses...