Having the Mind Flayers and Intellect Devourers mentioned in Deeps' answer, note that there are a couple of creatures (apart from Mind Flayer variants, like Alhoons and Illithiliches) that have identical or very similar abilities to the Tentacles and Mind Blast abilities of the Mind Flayers:

Having the Mind Flayers, Intellect Devourers and Elder Brains mentioned in Deeps' answer, note that there are a couple of creatures (apart from Mind Flayer variants, like Alhoons and Illithiliches) that have identical or very similar abilities to the Tentacles and Mind Blast abilities of the Mind Flayers:

Having the Mind Flayers mentioned in the previous answer, note that there are a couple of creatures (apart from Mind Flayer variants, like Alhoons and Illithiliches) that have identical or very similar abilities to the Tentacles and Mind Blast abilities of the Mind Flayers:

Having the Mind Flayers and Intellect Devourers mentioned in Deeps' answer, note that there are a couple of creatures (apart from Mind Flayer variants, like Alhoons and Illithiliches) that have identical or very similar abilities to the Tentacles and Mind Blast abilities of the Mind Flayers:

There are some new (and some older) spells and monster special abilities that requires intelligence saving throws

Apart from the spells in this answer the new spells that require intelligence saves are from Tasha's Cauldron of Everything:

• Mind sliver
• Tasha's Mind Whip

There are a handful of other monsters with such abilities, however there are very little connection between them and it's less likely that they play a huge enough role in an adventure, to pick proficiency in intelligence saves just for them.

There are some setting books and adventures where a usually powerful NPC has abilities that require intelligence saving throws even though they are not Mind Flayers or any other creatures described above. These NPC-s can be leaders of a cult, a country or something, the BBEG, a powerful mini-boss along the way or even friendly to the adventurers. In no specific order:

Having said all of that, the mere fact that we can count the intelligence saving throws in the game shows how rare they are. I haven't covered any UA, and I'm not aware of any psionic abilities of this sort that had became official.

There are some new (and some older) spells and monster special abilities that require intelligence saving throws

Apart from the spells in Deeps' answer the new spells that require intelligence saves are from Tasha's Cauldron of Everything:

• Mind Sliver
• Tasha's Mind Whip

There are a handful of other monsters with such abilities. However there is very little connection between them so it's less likely that they would play a huge enough role in an adventure to pick proficiency in intelligence saves just for them.

There are some setting books and adventures where a usually powerful NPC has abilities that require intelligence saving throws even though they are not Mind Flayers or any other creatures described above. These NPCs can be leaders of a cult, a country or something, the BBEG, a powerful mini-boss along the way or even friendly to the adventurers. In no specific order:

Having said all of that, the mere fact that we can count the intelligence saving throws in the game shows how rare they are. I haven't covered any UA, and I'm not aware of any psionic abilities of this sort that became official.