It's a play on Ottery St Catchpole, where the Weasley family (which she'll eventually marry into) live. Also, Ron's patronus is a टेरिए, टेरियर which is known for chasing otters.
Otter. Hermione learns how to conjure a patronus at the DA meetings in "Harry Potter and the Order of the Phoenix". It is the only spell that she has difficulty with
