A “loan shark” is a predatory person who offers financing at exorbitant rates. The term “shark” for such as person has been cited in print since at least 1713. The term “loan shark” was printed in the Topeka (KS) Daily Blade on February 22, 1876.
 
The International Monetary Fund (IMF) has been called the “loan shark of last resort” since at least 2011.
     
 
Wiktionary: loan shark
Noun
loan shark (plural loan sharks)
1. Someone who lends money at exorbitant rates of interest.
Synonyms
. usurer
 
Wikipedia: Loan shark
A loan shark is a person or body that offers loans at extremely high interest rates. The term usually refers to illegal activity, but may also refer to predatory lending with extremely high interest rates such as payday or title loans. Loan sharks sometimes enforce repayment by blackmail or threats of violence. Historically, many moneylenders skirted between legal and extra-legal activity. In the recent western world, loan sharks have been a feature of the criminal underworld.
 
(Oxford English Dictionary)
shark, n.1
Applied to persons, with allusion to the predatory habits and voracity of the shark; one who enriches himself by taking advantage of the necessities of others; a rapacious usurer, an extortionate landlord or letter of lodgings, etc., a financial swindler.
1713   R. Steele in Guardian 4 June 1/1   The Sharks, who prey upon the Inadvertency of young Heirs.
1804   Naval Chron. 12 249   The slopsellers, and other sharks, at this port.
 
loan-shark n. orig. U.S. (see quot. 1928   and shark n.1 2).
1905 B. L. Taylor & W. C. Gibson Log of Water Wagon 41   Loan sharks have been following the Lithia all day.
1911 Collier’s 4 Feb. 8/1   Mr. Ham became interested in the ‘World’s’ lucrative and lengthy list of loan-shark advertisements.
1913 Munsey’s Mag. Nov. 218/1   In New York the loan-sharks were doing a business of twenty million dollars per annum.
1913 Munsey’s Mag. Nov. 221/1   At the convention of the Legal Aid Society in Pittsburgh..the loan-shark evil was discussed.
1928 Daily Tel. 5 May 9/5   It is hoped by this plan virtually to put out of business the ‘loan shark’, who exacts usurious rates of interest from the person of small means.
         
22 February 1876, Topeka (KS) Daily Blade, “A Chance for a Hero,” pg. 2, col. 1:
He does not know that the Loan shark looks at his retreating form, as he walks away and smiles to himself as he says, “We’ve got him sure.”
   
15 April 1876, Chetopa (KS) Herald, “After the Battle,” pg. 2, col. 2:
The loan shark will ever be ready to loan you at three per cent a month. 
 
Chronicling America
2 March 1877, Buchanan County Bulletin (Independence, Iowa), “From Cono Township,” pg. 3, col. 5:
Neither are we unknown to the money loaning sharks that gather about such towns as Independence, looking after subjects as easy prey, who object not to security on our farms for money at 25 per cent interest.
     
Old Fulton NY Post Cards
14 December 1881, Truth (New York, NY), “Legal Tender,” pg. 2, col. 1:
... which would enable the money loaning sharks of Manhattan Island to corner the limited stock of gold, ...
 
3 September 1883, Chicago (IL) Tribune, pg. 2, col. 1:
WASHINGTON.
Judge Payson Still Engaged on the Record of The Texas Pacific Land-Grant.
The Southern Pacific Road Simply a Leased Line of the Central Pacific System.
Mr. Joslyn, of the Interior Department, Not Partial to the Money- Loaning Sharks.
 
25 January 1886, The Post Cincinnati, OH), pg. 2, col. 3:
John Kidd, formerly of Cincinnati, a loan shark of Indianapolis, has creditors interested $10,000 in him.
 
30 May 1886, Cleveland (OH) Plain Dealer, “Sam Short’s Sermon,” pg. 4, col. 7:
“But a speculator in grain, a real estate and small loan shark, or the wolf that builds up his bank account bychattel and real mortgage snaps, isn’t a philanthropist and you and I know it.”
   
9 October 1886, Minneapolis (MN) Tribune, “Condition of Trade,” pg. 7, col. 1:
This fact is being taken advantage of by the loan companies to secure by tax titles and other means such a claim upon their property that by the time it is properly improved to make it profitable, the aforesaid loan shark will quietly come forward and claim it.
 
15 February 1887, St. Paul (MN) Daily Globe, pg. 5, col. 3:
REMOVING THE CAPITAL.
A Scheme to Locate It in Kandyohi County—A Check on Convict Labor and Loan Sharks—Minneapolis Measures.
     
24 October 1888, St. Louis (MO) Republic, pg. 1, col. 6: 
A PITIFUL PLIGHT.
A Dakota Colony of Polish Jews Dying of Cold and Starvation.
(...)
Loan sharks lent them money and charged them exorbitant interest.
 
17 January 1891, The Constitution (Atlanta, GA), pg. 7, col. 1:
A LOAN SHARK.
A GEORGIA LADY THE VICTIM OF A NEW YORK SHARPER.
She Negotiates a Loan and the Accommodating Shylock Takes Advantage of Her and Seizes Her Household Goods.
A lady of a prominent Georgia family was outrageously victimized in New York the other day.
 
OCLC WorldCat record
A Widespread Form of Usury: The “Loan Shark”
Author: John Lisle
Edition/Format:   Article : English
Publication: Journal of the American Institute of Criminal Law and Criminology, Jul., 1912, vol. 3, no. 2, p. 167-171
Database: JSTOR
 
OCLC WorldCat record
The anti-loan shark bank operating the Morris plan.
Author: Atlanta Loan and Saving Co.
Publisher: Atlanta, The Co. [©1912]
Edition/Format:   Book Microform : Microfilm : Master microform : English
 
OCLC WorldCat record
The loan shark in Chicago : a survey made by the Bureau of Social Surveys, Department of Public Welfare, City of Chicago.
Author: Chicago (Ill.). Dept. of Public Welfare. Bureau of Social Surveys.
Publisher: [Chicago : Dept. of Public Welfare, 1916]
Series: Bulletin of the Department of Public Welfare, v. 1, no. 4 (November 1916)
Edition/Format:   Book : English
 
OCLC WorldCat record
Quick cash : the story of the loan shark
Author: Robert Mayer
Publisher: DeKalb, Ill. : Northern Illinois University Press, ©2010.
Edition/Format:   Print book : English
Summary:
In this intriguing and accessible book, Mayer presents the history of payday lending using the colorful (and sometimes criminal) city of Chicago as a case in point. With an eye to the future, Mayer also aptly assesses the consequences of high-interest lending - both for the people who borrow at such steep prices and for society as a whole.-publisher description.
 
23 May 2019, Wall Street Journal

It is more likely that the English word “shark” originally referred to our own species. It may be related to Schurke in German (and schurk in Dutch), referring to a scoundrel or villainous person. If that’s the case, then the dangerous fish first got its name by analogy to predatory humans, rather than vice versa.
 
Regardless of which kind of “shark” came first, the word labeled various types of swindlers by the late 16th century. A speech by Sir Thomas More, adapted for the stage by William Shakespeare, told of “ruffians” who “would shark on you,” as “men like ravenous fishes would feed on one another.”
(...)
In the London newspaper the Guardian in 1713, Richard Steele warned of “the Sharks, who prey upon the Inadvertency of young Heirs.” That sense of “shark” flourished in American colloquial usage. The phrase “loan shark” appears as early as 1876 in the Daily Blade of Topeka, Kan., in a context that might pass muster with Mr. Sanders and Ms. Ocasio-Cortez. An editorialist depicts a Kansas farmer who confidently “walks up to the counter of some Loan and Trust Company” but is victimized by a “Loan shark” charging high rates based on his income.

Zimmer, Ben. Wall Street Journal (Online); New York, N.Y. [New York, N.Y]23 May 2019.