Rhino is an old slang term for money. It often occurs in the phrase ‘the ready rhino’, which means hard cash. There is no agreement about its origins. Some people have pointed to the fact that rhino horn was a very expensive commodity in ancient pharmacies while others link it to expressions like ‘paying through the nose’ (rhino is Greek for nose).
The liar Daniel Cassidy, in his ridiculous contribution to the sum of human ignorance, How The Irish Invented Slang, claimed that it derives from the Irish word rianú. He claims that this means engraving and that therefore this refers to the engraved patterns on coins. Leaving aside the pronunciation, which is not that close, and the fact that this word is a verbal noun and much more likely to describe an action or a process than an object, rianú is defined as ‘marking, tracing, delineation’ by Ó Dónaill and strangely enough, Dinneen doesn’t give the meaning of engraving either! Even if it did mean engraving, there would be a major problem with this claim. Cassidy seems to think that craftsmen individually engraved the designs on each coin! Of course, since Roman times, coins have been made by placing a disk of soft metal between two dies and striking them to stamp the image from the die onto the metal, not by engraving.
The origins of the term rhino for money are unknown, but the possible connection between the word rianú and coins is so weak and so contrived that it really isn’t worth considering.