[At Marina's funeral] D'Onsky's son, a person with only one arm, threw his remaining one around Demon and both wept comme des fontaines. (Ada: 3.8)

In a letter to his wife (born Ekaterina Raevski) M. F. Orlov writes that Pushkin умеет ездить только на Пегасе да на донской кляче (can ride only Pegasus and a Don mare; Veresaev, Pushkin in Life, p. 167). M. F. Orlov and one-armed prince (Alexander Ypsilanti) are mentioned by Pushkin in a 1821 poem addressed to Vasiliy Davydov* (see EO Commentary, vol. III, p. 331):

While General Orlov,

Hymen's recruit with shaven head...

and on the Danube's bank to drown his grief

our one-armed prince stirs strife...

As I pointed out before, Onegin's Don stallion and the one-armed prince are mentioned in Eugene Onegin:

but since to the back porch

there was habitually brought

a Don stallion for him (Two: V: 2-4)

the one-armed prince to the friends of Morea

from Kishinev already winked (Ten: IX: 3-4)

a propos des fontaines: The Fountain of Bahchisaray is mentioned in The Fragments of Onegin's Journey:

Was I like that when I was blooming?

Say, Fountain of Bahchisaray! ([XIX]: 5-6)

*In a 1836 poem, written a few months before his fatal duel with d'Anthes and addressed to another Davydov, Denis ("To you, the bard, to you, the hero!"), Pushkin confesses that he was a rider of tame Pegasus (naezdnik smirnogo Pegasa).

Denis Davydov = syn Davidov + de = sny + Avidov + ded

Denis + Pegas = penis + Degas (syn Davidov - "son of David," Jesus Christ; de - la particule; sny - dreams; Avidov - Baron Klim Avidov; ded - grandfather; Pegas - Pegasus; Degas - French painter)