It must be a natural thing for the brain to do by letter association, because it just did that to me as well. Very odd double take.
The reason is efficiency and it's the same reason we can read faster than, for example, a child learning to read phonetically, sound by sound. We don't read letter by letter but instead by a first-glance impression based on the first and last few letters of a word as well as expectancies about context (both of the sentence and the greater context) and word frequency. If any confusion or uncertainty arises, a second more elaborate process is prompted, hence your double-take.
'Monkey Finger' probably makes more semantic sense than 'Monkey Flinger' and would likely be a more frequently used phrase, so the brain took the first letter and last few letters and decided on finger. Don't worry, it just means your brain is saving energy
In case you haven't seen the famous example of this phenomenon in action.....
Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.
