yep, a speedy rabbit if you've got one. Snails are also popular but not quite as versatile as rabbits. Squirrels are gaining in popularity too. All else fails, take a look through your pet journal & see if you have any pets that can't be traded or tamed. Those are usually my #1 priority for upgrades, because there's absolutely no way to get a better one.
xoxo, Your Pet Battling BFF, Tamer Liopleurodon.