Don't know why on 8d, but the OED lists for rhino, "Money. (Often ready rhino.)" No explanation given as to where the term comes from.
from Brewer's:
Rhino
Ready money. (See Nose.) May not this explain the phrase �paying through the nose� (par le nez), that is, paying ready rhino. Rhino = money is very old.
Some, as I know,
Have parted with their ready rhino.
8d the slang dictionary I have says about it's etymology:
[17th Century] money, thus [19th Century] rhino-fat meaning wealthy
etymology unknown,; one suggestion , that it refers to the rhinoceros, then a fabulous creature 'worth it's weight in gold', implies a certain lexicographical desperation; the term moved from underground to general slang in mid-19th Century.