According to Bess (which is 'fiend' in Russian), Dan's buxom but otherwise disgusting nurse, whom he preferred to all others and had taken to Ardis because she managed to extract orally a few last drops of 'play-zero' (as the old whore called it) out of his poor body, he had been complaining for some time, even before Ada's sudden departure, that a devil combining the characteristics of a frog and a rodent desired to straddle him and ride him to the torture house of eternity. To Dr Nikulin Dan described his rider as black, pale-bellied, with a black dorsal buckler shining like a dung beetle's back and with a knife in his raised forelimb. (Ada: 2.10)

The name Nikulin, of uncle Dan's last doctor (grandson of the great rodentiologist Kunikulinov*), rhymes with Nulin (in fact, Nikulin = ik** + Nulin). The eponymous hero of a poem (1825) by Pushkin, Count Nulin turns out to be a horse in Chekhov's story Uchitel' slovesnosti ("The Teacher of Literature," 1894) which begins:

There are also dogs (cf. everything had gone to the hell curs, k chertyam sobach'im) in Chekhov's stroty: There were so many house-dogs and yard-dogs that he had only learnt to recognize two of them in the course of his acquaintance with the Shelestovs: Mushka and Som. Mushka was a little mangy dog with a shaggy face, spiteful and spoiled. She hated Nikitin: when she saw him she put her head on one side, showed her teeth, and began: "Rrr . . . nga-nga-nga . . . rrr . . . !" Then she would get under his chair, and when he would try to drive her away she would go off into piercing yaps, and the family would say: "Don't be frightened. She doesn't bite. She is a good dog." Som was a tall black dog with long legs and a tail as hard as a stick. At dinner and tea he usually moved about under the table, and thumped on people's boots and on the legs of the table with his tail. He was a good-natured, stupid dog, but Nikitin could not endure him because he had the habit of putting his head on people's knees at dinner and messing their trousers with saliva. Nikitin had more than once tried to hit him on his head with a knife-handle, to flip him on the nose, had abused him, had complained of him, but nothing saved his trousers.

The name Nulin comes from nul', "zero" (cf. 'play-zero,' Bess's pun on plaisir, which also exists in Russian and is sometimes spelled blezir***). As to Nikitin, the hero of Chekhov's "The Teacher of Literature," his name comes from Nikita. One is reminded of Pushkin's Czar Nikita (the father of forty almost impeccable daughters), the Soviet leader Nikita Khrushchyov, and Nikita, the watchman with huge fists in Chekhov's stroy Palata № 6 ("Ward 6," 1892):

The watchman Nikita, an old soldier wearing rusty good-conduct stripes, is always lying on the litter with a pipe between his teeth. He has a grim, surly, battered-looking face, overhanging eyebrows which give him the expression of a sheep-dog of the steppes, and a red nose; he is short and looks thin and scraggy, but he is of imposing deportment and his fists are vigorous.

In the same letter of November 25, 1892, to Suvorin, in which he accuses the Muse of having a flat spot under her skirt and complains of the absence of alcohol in the books of contemporary writers, Chekhov modestly compares his Ward 6 to lemonade. I'm afraid the three Nikitin-to-Nikulin anagrams below are non-erotic either, nor would they intoxicate anybody:

s kulakami - Russ., with fists (according to S. Kunyaev, dobro dolzhno byt' s kulakami, "good should have fists"); kulak - Russ., fist; kulak; Tatar, ear; Kulak ("The Middleman") is a poem (1858) by Ivan Nikitin (who is mentioned in The Gift); the term 'Vandemonian' is glossed as Koulak tasmanien d'origine hollandaise in Pompier's cheap novel (2.5); kulak = kukla (doll) = Lukka (Lucca, a Roman spa; Paoline Lucca, the famous operatic soprano mentioned in Tolstoy's Anna Karenin)

*Oryctolagus cuniculus is the Latin name of European/Common rabbit; most of the physicians in Ada bear names connected with rabbits; the polygamous hero and narrator in Chekhov's story Noch' pered sudom ("The Night before the Trial") is mnimyi doktor Zaitsev (the imposturous "Dr Hare")