The character who uses the the expression "take time by the fetlock" is one N. Winkle. The full quote is:
Originally Posted By: N. Winkle
Mr. Pickwick, Sir:--
I address you upon the subject of sin the sinner
I mean is a man named Winkle who makes trouble in his
club by laughing and sometimes won't write his piece in
this fine paper I hope you will pardon his badness and
let him send a French fable because he can't write out
of his head as he has so many lessons to do and no brains
in future I will try to take time by the fetlock and
prepare some work which will be all commy la fo that
means all right I am in haste as it is nearly school
time.
Yours respectably,
N. WINKLE


As we can see his command of French is a little less than notable and he gives punctuation very short shrift. I suspect that his use of fetlock in this context is very much a malapropism.