What do I do with?

What is it worth?

How rare is it?

Sorry, probaly a stupid question, but I just got my first one.

It's a single use item that will randomly change one gene into another.

In auctions it will generally sell for at least 3k and it is one of the rarer items.

Those are somewhat rare (but not quite as rare as, let's say, rainbow jars), and it's pretty easy to quickly use up a lot of them. Especially if you end up with some gene going back and forth between good and bad. 

I'd suggest the minimum of 10k buyout for those. Mutagen is in pretty high demand, and I certainly buy those myself at that price.

I don't recommend using them on just anything. More often than not, you'll only end up with a worse gene. Especially for species such as Throdama, that have more bad than good genes. Myself, I generally use them when I have hard time deciding whether I should keep or pound a pet; if it ends up with better genes, then I keep it.