A quick tip about breeding animals, if you train a child animal it will become domesticated, permanently tame, and never need to bother your animal trainers again. However, several animals, especially cave animals, will be "born" as adults, which means they can never be tamed. I know for sure that Giant Toads are such creatures, and I think Giant Cave Spiders are too, but I've never caught two at once. So compared to a Cave Crocodile or an Elk Bird, which can be fully domesticated, Giant Toads may not represent the best use of your animal trainers' time. It also costs meat to tame them.
A breeding population of non-domesticatable animals may overwhelm your animal trainers, especially if unexpected catastrophes knock your trainers out of action. Likewise, if you run out of meat to train them with. This can result in training decay, eventually causing the Giant Toads to go wild again and attack your dwarves, and Giant Toads are plenty dangerous enough to kill a couple dwarves before going down. (But you can also use this mechanic to your advantage in military applications)
Which is not to say there is no benefit to raising Giant Toads. They can be a useful source of meat and as they said, high value bones and leather, or you can pasture them near your entrances to chase off (or more accurately fight off) other dangerous cavelife. To breed them, they need a source of water. If you let them wander around the fortress, they're going to spend their time cruising your dining rooms and statue gardens instead of making tadpoles, so make sure you pasture them in an area with water. The same is true for Giant Olms if you encounter them.
Another odd thing I've never really experimented with is that dwarves outside of your fortress do not suffer these penalties to domestication. I think it's possible that you can trade your trained Giant Toads to caravans (not the wild ones) and they may one day trade back a fully domesticated Giant Toad. I also think if you see the message "the dwarves of such-and-such are now expert Giant Toad trainers" your civ will be able to embark with Giant Toads, although exotic animals cost insane amounts of starting points. I assume that if that works, it would also work with the GCS.