Or maybe Jake did know but never told Finn.he isn't exactly known for being totally up-front. Why he didn't know Tiffany was a human, I don't know - maybe Tiffany was raised by another species like Finn was and therefore wasn't immediately recognizable. Why did Jake choose Tiffany to be his 'new best friend'? Because he can detect that Tiffany is extremely similar to Finn, only evil, either because they smell almost the same (dogs have incredible senses of smell) or because of some magical intuition.

Additionally, the reason Tiffany is so evil is that he is constantly jealous of Finn and subconsciously feels like he should be in his brother's place (sibling rivalry), but instead he is constantly overlooked and never taken seriously. As suggested in a different WMG, Finn inherited his mother's heroic tendencies and good-heartedness, but Tiffany instead inherited Martin's 'jerkiness' and selfishness. In 'Dentist', Tiffany and Finn were shown to work well together with similar moves and mannerisms, as well as Finn saying numerous times that they are not so different. Tiffany's skin is oddly colored due to mutation caused by the nuclear fallout of the Mushroom War. Tiffany was Finn's brother.On the most shallow level, they are both human-shaped and blonde.

Gunter adventure time