Pre-existing conditions are typically not covered by pet insurance coverage. A pre-existing condition, on the other hand, will never prohibit you from getting pet insurance. Your coverage possibilities for that condition will be determined by whether or not your pet's pre-existing condition is curable.
The information in this answer is general in nature and is not legal or insurance advice for your specific situation. Policy language varies carrier by carrier; your declarations page is the final word. If you're unsure, call your agent before you need to file.