What part of speech is round in the sentence The horses must this corner?

The sentence is incomplete. We need to know what the horses are doing with the corner.

However, based on the provided information, "round" would most likely be an adjective.

Here's why:

* Adjectives modify nouns. In this case, "round" would be describing the corner, making it a "round corner."

Possible completions of the sentence:

* The horses must round this corner. (verb)

* The horses must go round this corner. (adverb)

* The horses must turn the round corner. (adjective)

Let me know if you have the full sentence, and I can give you a more definitive answer!