Are Sylesti spawns completely random or are certain species more likely to show up in certain areas?
Currently I'm looking for a male Loch Ness Monster (whatever the name is.)

Different sylesti spawn in different regions, hopefully someone else can tell you which is where because I don't quite remember.

I do know that a lot of Sylvorpa and Kelpari spawn in the third region (they're water creatures so they are all over the giant lake there). But if you would like to not wait, I'm sure there are some for sale in the advanced search. On some pages like the forum, it is located at the left of the page, and on others it is at the top. You can search for Specific species and order them by cheapest to most expensive.

Hope that helps! and Welcome to Sylestia!