I have a soft spot for training up unconventional pets and have stables full of trained dogs, bears, wolves, walrus etc. But my favorite without a doubt is the Giant Rat. The start stats are not that far off from the Giant Toad and is also 1-3 slots. So this should give you a rough idea of what you could do with it. Not sure what specials you may or may not get though, not trained a frog yet.
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Tackles Ancient Wyrms, Balrons etc with a bit of healing. Defense wise a 3 slot can be as good as a 5 slot, damage is where it falls behind a bit but fun none the less.