It is a partnership with a lion when all the profit goes to one person and the others are cheated out of their shares by force. The phrase occurs in the Pandects, in which Aristo (according to Ulpian) reports that Cassius gave his legal opinion that a partnership could not be entered into such that one partner got only profit, and the other partner only loss, and he used to call such a partnership "with a lion". But it is clear that Cassius' expression, surely a proverbial one, originated in a fable of Aesop the Greek, which goes something like this.
A lion, an ass, and a fox entered into a partnership whereby they would share in common whatever they caught by hunting. When they got their prey, the lion ordered the ass to divide it. Stupid as he was, the ass divided it into three equal parts. Wherefore at once the lion, outraged that he was made equal with the others, attacked the ass and tore him to pieces. The fox was left; the lion ordered her to make the division anew; she gave almost all of the prey to the lion, keeping for herself only a few meager scraps. The lion approved the division and asked the fox who had taught her the art of dividing. The fox answered, "The fate of the ass."
Leonina societas est, cum omne commodum ad unum aliquem redit, reliquis vi fraudatis. Exstat in Pandectis, in quibus ex Ulpiano refert Aristo, Cassium respondisse, societatem talem coiri non posse, ut alter lucrum tantum, alter damnum sentiret, et hanc societatem leoninam solitum appellare. Ceterum Cassianam appellationem, haud dubium proverbialem, ex Aesopi Graeci apologo natam apparet, qui talis circumfertur.
Leo, asinus, et vulpes societatem inierant, ut quod venatu cepissent, id in commune partirentur. Praedam ubi erant nacti, leo iubet, ut asinus partiatur. Ille, ut est stolidus, in tres aequas portiones distribuit. Qua gratia mox indignatus leo, quod ceteris aequaretur, asinum adortus dilaniat. Restabat vulpes, eam de integro partiri iubet, illa totam ferme praedam leoni attribuit, sibi vix paucula quaedam servans. Leo, comprobata distributione, rogat, quisnam illam artem partiendi docuisset. Et vulpes, Calamitas, inquit, asini.
On Aristo, Ulpian, and Cassius see Justinian, Digest 184.108.40.206
. The fable of Aesop
is 260 in Halm's numbering. For other versions of this fable (including Babrius 67 and Phaedrus 1.5) see here