Question: What animal with only four extant species was the name borne by four ships of the Royal Navy

Context paragraphs:
Hyena: Hyenas or hyaenas (from Greek ὕαινα "hýaina") are any feliform carnivoran mammals of the family Hyaenidae .  With only four extant species, it is the fifth-smallest biological family in the Carnivora, and one of the smallest in the class Mammalia.  Despite their low diversity, hyenas are unique and vital components of most African ecosystems.

C-class cruiser: The C class was a group of twenty-eight light cruisers of the Royal Navy, and were built in a sequence of seven groups known as the "Caroline" class (six ships), the "Calliope" class (two ships), the "Cambrian" class (four ships), the "Centaur" class (two ships), the "Caledon" class (four ships), the "Ceres" class (five ships) and the "Carlisle" class (five ships).  They were built for the rough conditions of the North Sea, and proved to be rugged and capable vessels, despite their being somewhat small and cramped.

Malta-class aircraft carrier: The "Malta"-class aircraft carrier was a British large aircraft carrier design of World War II.  Four ships were ordered in 1943 for the Royal Navy, but changing tactical concepts, based on American experience in the Pacific War, caused repeated changes to the design, which was not completed before the end of the war.  All four ships were cancelled in 1945 before they were laid down.

HMS Bridgewater: Four ships of the Royal Navy have borne the name HMS "Bridgewater", while one ship of the navy of the Commonwealth of England has also carried the name:

HMS Hyaena: Four ships of the Royal Navy have borne the name HMS "Hyaena" (or HMS "Hyæna"), after the Hyena, a family of carnivorous mammals.  Two others were planned but either commissioned under another name or cancelled.

C and D-class destroyer: The C and D class was a group of 14 destroyers built for the Royal Navy in the early 1930s.  As in previous years, it was originally intended to order a complete flotilla comprising eight destroyers—plus a flotilla leader as the ninth unit—in each year.  However, only four ships—plus a leader—were ordered under the 1929–30 Programme as the C class.  The other four ships planned for the C class were never ordered as an economy measure and disarmament gesture by the Labour government of Ramsay Macdonald.  A complete flotilla—the 'D' class—was ordered under the 1930–31 Programme.

Town-class cruiser (1910): The Town class was a group of twenty-one light cruisers built for the Royal Navy (RN) and Royal Australian Navy (RAN).  These vessels were long-range cruisers, suitable for patrolling the vast expanse covered by the British Empire.  These ships, initially rated as second class cruisers, were built to a series of designs, known as the "Bristol" (five ships), "Weymouth" (four ships), "Chatham" (three RN ships, plus three RAN ships), "Birmingham" (three ships, plus one similar RAN ship) and "Birkenhead" (two ships) classes – all having the names of British towns except for the RAN ships, which were named after Australian cities.

Linuparus: Linuparus is a genus of spiny lobsters in the family Palinuridae.  It contains four extant species, and 32 fossil species, ranging from the Early Cretaceous to the Oligocene. " L. trigonus" is the only extant species also known from the fossil record.

Mangrove horseshoe crab: The mangrove horseshoe crab ("Carcinoscorpius rotundicauda") is a chelicerate arthropod found in marine and brackish waters.  Despite its name, it is more closely related to spiders and scorpions (all are in the subphylum Chelicerata) than to crabs.  It is the only species in the genus Carcinoscorpius.  The mangrove horseshoe crab occurs only in Asia and can be found in India, Indonesia, Malaysia, the Philippines, Singapore, Thailand and Hong Kong.  There are four extant species of horseshoe crab.  The biology, ecology and breeding patterns of "C. rotundicauda" and the two other Asian horseshoe crab species, "Tachypleus gigas" and "Tachypleus tridentatus", have not been as well documented as those of the North American species "Limulus polyphemus".  All four extant species of horseshoe crabs are anatomically very similar.

HMS Port Royal: At least four ships of the Royal Navy have borne the name HMS "Port Royal", after the British naval base Port Royal in Jamaica:

Supporting paragraphs: HMS Hyaena Hyena
Answer: Hyena