This depends on your playstyle and the way you want to play.
Would the characters' reasonably know about these creatures and is that something you want to have the players' have access to as part of play?
Now, there's plenty of old school play where you pretty much know nothing about a creature until you've dealt with it enough to figure out what it can do. This works either where the monster is rare and the characters would know nothing about it, or in videogames where player skill is created through repeatedly encountering the same thing over and over until you gain understanding of what it is about.
Another alternative is to give the description based on the characters' understanding - presumably the druid knows about many animals, the necromancer knows about ghosts, etc. and can tell the differences between the well known types and has a good idea about what their abilities are. This is where characters having appropriate skills or backgrounds can be very useful.
You can scale the description up or down, accordingly. The average person figures out that it's an animated statue. The cleric says, "No, this is a golem, a divine secret that should have been used to protect a place." The character who can cast the "create Golem" spell probably knows enough that you might as well give them the stats for the average golem, and so forth.
Also be aware that players may see this as a game like many people see videogames - of course you look up the stats on the monsters you face. Some players might just love the lore and look up stuff on the monsters because they're cool and fun. So the whole point of mysterious monsters usually drops away quickly for most gamers - you then end up with the second problem of what happens when players begin using information their characters couldn't possibly know? Do you force players to play suboptimally, or do you accept it like how when someone replays a videogame they already can speed through because they know what to expect?
There's no one right answer, there's just what sounds fun for you and your group and it's good to lay out those expectations up front.