Sorry to rain on the parade, but the essential difficulty for all advocates of these large, mythical creatures is simple biology.

For them to survive at all, there must be a significant population available for breeding. This population must be dense enough for individuals to find one another reasonably effectively (unless one argues that they reproduce by parthenogenesis). Large mammals that are usually solitary by habit (the common descriptor of this legend) have to leave substantial clues at breeding time so that they can find each other.

Relict populations of large mammals tend to die out pretty rapidly precisely because of this lack of a viable population.

It's the same for the Loch Ness monster. To paraphrase, there cannot be only one.