He looks like a Common Pleco, IMO
. Your 56g isn't even close to large enough for him. A Common Pleco needs a 90g minimum and 125g is better. They average 22-24" and have a HUGE bio-load compared to most other fish. Keeping them in a tank that is to small will cause stunting (warped spine) and internal problems, which will cause pain and shorten their life.
Common Plecos are very cool and can be incredibly friendly and they can be trained. But they aren't for the average fish keeper, because of their size and tank requirements.
There is a Pleco Profile thread here on AA. They can give you all the details you'd need.
If you are getting a much bigger tank, then by all means Common Plecos make great (fish) pets. Otherwise, be kind to him and find someone with the tank size he really needs.
If you still want a Placo for your tank there are so many that are much smaller and will be better suited to your 56g tank.
It's true that a lot of the smaller Plecos can be a little pricey, but not all of them. What you find online is just a small sample of how many cool small Plecos there really are.
I just recently went into my LFS
and asked them if they had any cool Plecos that stayed under 6" and didn't cost very much. We've all heard of BN Plecos, but he had these really awesome Green Dragon BN Plecos for $8.99 each. They are beautiful, only grow to 5" long and you could have 2-3 of them in your tank, depending on your other stock. They are just one example of whats out there, if you do a little research.
As far as this Common Pleco being given away. Some FS will will re-home Common Plecos for you. You just need to make a few phone calls and take him to the store.
I've re-homed a few of fish this way. Including 2 CP after I found out how big they got. And how much space they need. The FS is better equipped to house the Pleco while waiting for an appropriate home.