Alright, so when I go to challenge an opponent and try to choose my battle pet from the dropdown list, my pet's name isn't there. He's at the resort and fed, as all of my pets are, so I can't think of any reason as to why he wouldn't be viable for battling. I thought that it might be because he's my active pet, so I tried switching the active pet and trying again. Still no dice.

Are there any other known, non-glitch reasons as to why I can't use my battle pet? Is anyone else having this trouble?

Is Evearto your battle pet? If so, did you remember to heal its HP? That pet is currently at 0/92HP. http://subeta.net/petinfo.php?petid=849732

If you heal it, it's fully fed, happy, wasn't transferred to you in the past 30 days and it still doesn't show up in your drop-down menu, check the statistics page to make sure you're not already in a battle. If you are, either enter the battle and finish it, or forfeit.

Well, don't I feel embarrassed? Thank you very much for pointing that out! Evearto is able to battle now. ^_^;