I don't know if and where there are Mellotron samples in Logic, but if you're really into these kind of sounds, I recommend looking into M-Tron Pro by gforcesoftware and SampleTron by IK Multimedia. I do think they both come as plugin AND stand alone versions and offer demos to try before you buy.
Of course you have to shall out some extra shekels, but if you really like these "instruments" and sounds, they're worth it, both IMO.
Right, the Mellotron by definition already was a kind of primitive sampler. I just know that I heard some great Mellotron sounds in a slightly earlier version of Logic but now I don't see them. You mentioned hundreds of Mellotron sounds. Which ones were you referring to?
I don't know of any specific sounds in EXS24 that were sampled from an original Mellotron tape. M-Tron Pro by GForce is excellent. It has a nice collection of what was available for the Mellotron decades ago.
Here's a neat history:
If you check the link to G Force you will see what I mean. I wasn't referring to anything in Logic just pointing out that there is not a sound called mellotron but a whole lot of them and they are all there. It's built around the original samples with all the tape hiss and sligtly "out of tune-ness" that I personally love.
I suppose that the mellotron signature sound for many people would be the flute which basically is just a noisy flute but if you play the right chords will imediately take you down and going to Strawberry Fields.
If you like this kind of thing I cannot recommend the M-Tron enough and the recent Pro version makes it even better (although heavier on dsp).
If I remember correctly, Arturia has some nice Analog Mellotron sounds in their "Analog Factory", "ARP 2600" or "Moog Modular V" plugin software. (www.arturia.com)
I haven't used these all that much but I think I remember it there in one of their plugins I have.