I have a tadpole that just won't turn into a frog?

About 6 months ago I was hiking and I stumbled across a bunch of tadpoles so I decided to take about 20 of them. Well since then they have all turned into American Toads and Pacific Tree Frogs, but there is this one tadpole that just will not turn into a frog. I have had it since the rest of them and it is the only one left that hasn't turned into a frog. It has it's back legs but doesn't not have his front. The only thing I can think of is that he is a different species from the rest but the only tadpole I know of taking that long to metamorphose is a Bullfrog but Bullfrog tadpoles are big and this guy is small. What do you think?