Apparently, monsters can’t lie. They cannot convey what we believe to be a lie. However, that does not mean they can’t fool people. Omitting facts, for example, or leading them to believe something by revealing only specific facts. However, in case of contracts, lying through the aforementioned methods becomes essentially impossible. Well, if one can talk oneself out of the lie then no problem; but if one publicly declares a lie, one’s existence will be erased. It’s a rule pertaining to every monster.
However, monsters born through birth are not always bound by this rule, and sometimes capable of telling lies. Goblins can lie freely. Only monsters of spontaneous birth have trouble lying. On the other hand, high monsters and demons exemplify this rule, and are famous for being bound by contracts. Demons don’t tell lies. Although they are often called sly, they are actually a rather pure race.
Pledge magic is often used to prevent that, on top of swearing upon one’s very existence.