TotW

I believe Tessie didn't show itself to the researchers because of the way Tessie's cousin Nessie (Loch Ness Monster) was treated. Tessie saw that when Nessie was discovered it attracted people from around the world trying to study and possibly capture Nessie for profit. The humans, must've captured Nessie and studied it intently. However, in the process of studying Nessie and taking samples, they must've destroyed the bones. This can possibly be supported by the fact that in Fourside, all of the bones in the museum are replicas Tessie, being an intelligent animal understood that humans were the cause of this and vowed not to get close to humans seeing the danger they posed to her. Also, it also can be proven because of the hotel employee who bought the wrong newspaper in Twoson, the headline was about Tessie and Nessie being cousins. Also, you don't ever see any of Tessie's other relatives assuming they didn't die off from natural causes which could mean they were also captured by scientists.

However, when Jeff came along, she saw how Jeff treated Bubble Monkey by giving him a piece of gum. So when Bubble Monkey called her, she trusted Jeff enough to allow him to ride her across the waters.