Make sure to check with the airlines if you're flying into Mexico. Often airlines have restrictions on size, weight, type of carrier, etc. Be sure to contact your airline and plan ahead.
Also, make sure to bring along extra pet food especially if your pet has specific needs or is particular about a certain brand. Most large cities in Mexico have big grocery stores like Wal-Mart, Costco and Smart and Final, that do have a variety of pet foods.
Another good idea is to make sure that your pet is wearing an ID tag with all of your necessary information at all times.