Zen
I'm not much of a writer but I wanted to offer you some insight into working in a small animal clinic. Maybe some of this will help in composing your cover letter.
Clinic work is fast paced and often hectic. The ability to multitask is a must. Handling a variety of people in a pleasant manner is also a desired quality. Make sure your voice "smiles" when you talk to the office manager or person conducting the interview. Think about how you would handle five clients standing around the front desk, each wanting to be taking care of ASAP - you will probably be asked how to handle a situation like this.
As far as working with animals here are a few things to consider. Does your disability limit how much you can lift? Assistants are often asked to help prep. animals for surgery, assist with x-rays, etc so you may be asked to help lift a 140 lb dog up onto a table, would you be able to do this? Does working with fecal material or vomit (which sometimes smells awful) bother you? How about needles or blood, would these bother you? A question almost always asked during an interview is how do you feel about euthanasia? You will at some point be asked to prepare an owner for this (sign them in, have them sign consent forms, make arrangements for the body, etc) or as an assistant help the vet or vet tech in the actual procedure. Do you have a desire to learn how to restrain animals properly, run a variety of tests, assist the vets in surgery or filling prescriptions? These may be things to point out in your letter.
Not sure if any of this is helpful. I hope you get the job. I loved working in a clinic but after 15 years of working two jobs I simply reached a point of burnout and I decided to quit my part-time clinic job. I learned an awful lot in the years I worked for a local clinic and hope to someday return to a clinic job. Good luck and keep us posted.