Exhibit 5.1

101 Second Street 18th Floor San Francisco, CA 94105 +1 415 543 8700 Fax +1 415 391 8269 reedsmith.com

May 18, 2015

Jaguar Animal Health, Inc.

185 Berry Street, Suite 1300

San Francisco, California 94107

Ladies and Gentlemen:

We have acted as counsel to Jaguar Animal Health, Inc., a Delaware corporation (the “Company”) in connection with the filing by the Company on the date hereof of a Registration Statement on Form S-8 (the “Registration Statement”) with the Securities and Exchange Commission (the “Commission”) covering the offer and sale of up to 1,129,003shares of the Company’s Common Stock, $0.0001 par value per share (the “Shares”), issuable pursuant to its 2014 Stock Incentive Plan (the “2014 Plan”) and 2013 Equity Incentive Plan (the “2013 Plan”, and together with the 2014 Plan, the “Plans”). This opinion is being furnished at the Company’s request in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act of 1933, as amended, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or the prospectus forming a part thereof, other than as to the issue of the Shares.

In rendering the opinion hereinafter expressed, we have made such legal and factual examinations and inquiries, including an examination of originals or copies certified or otherwise identified to our satisfaction as being true reproductions of originals, of all such documents, records, agreements and other instruments, including the Registration Statement, the Plans, the Certificate of Incorporation, as amended, of the Company, the Bylaws of the Company, and corporate minutes of the Company as we have deemed necessary and appropriate for the purpose of this opinion. We have assumed that there are no agreements or understandings between or among the Company and any participants in the Plans that would expand, modify or otherwise affect the terms of the Plans or the respective rights or obligations of any participants thereunder. We have further assumed the genuineness of all signatures, the authenticity of all documents, certificates and records submitted to us as originals, the conformity to original documents, certificates and records of all documents, certificates and records submitted to us as copies, and the truthfulness of all statements of fact contained therein.

Based upon the foregoing, and having regard to legal considerations and other information that we deem relevant, we are of the opinion that, as of the date hereof, when the Shares shall have been duly registered on the books of the transfer agent and registrar therefor in the name or on behalf of the purchasers, and have been issued by the Company against payment therefor in the circumstances contemplated by the Plans and assuming in each case that the individual issuances, grants or awards under the Plans are duly authorized by all necessary corporate action of the Company and duly issued, granted or awarded and exercised and paid for, for consideration at least equal to the par value thereof, in accordance with the requirements of law and the Plans (and the agreements and awards duly adopted thereunder and in accordance therewith), the issue and sale of the Shares shall have been duly authorized by all necessary corporate action of the Company and, when and to the extent that the Shares are issued in accordance with the foregoing, such Shares will be validly issued, fully paid and non-assessable.

We express no opinion regarding the effectiveness of any waiver (whether or not stated as such) contained in the Plans of rights of any party, or duties owing to it, that is broadly or vaguely stated or does not describe the right or duty purportedly waived with reasonable specificity or any provision in the Plans relating to indemnification, exculpation or contribution.

NEW YORK ¨ LONDON ¨ HONG KONG ¨ CHICAGO ¨ WASHINGTON, D.C. ¨ BEIJING ¨ PARIS ¨ LOS ANGELES ¨ SAN FRANCISCO ¨ PHILADELPHIA ¨ SHANGHAI ¨ PITTSBURGH ¨ HOUSTON SINGAPORE ¨ MUNICH ¨ ABU DHABI ¨ PRINCETON ¨ NORTHERN VIRGINIA ¨ WILMINGTON ¨ SILICON VALLEY ¨ DUBAI ¨ CENTURY CITY ¨ RICHMOND ¨ GREECE ¨ KAZAKHSTAN

We express no opinion herein as to the laws of any state or jurisdiction other than the General Corporation Law of the State of Delaware.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and the reference to this firm therein. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission.

Very truly yours,

/s/ Reed Smith LLP

Reed Smith LLP