ARE, not were, for me.
Across 17 characters in the 2 campaigns I currently DM in 3.5, only one is not a PHB race. A tiefling NPC they adopted - actually Karlach, who I converted from BG3.
In the past:
(1) In a one shot, someone played a centaur. Since it was outdoor exploration, it was fine and fun for everyone as an early experiment.
(2) A first-time player (long-time novel reader) joining late in a campaign took over a rescued prisoner NPC, I think Tiefling, rather than make their own character. They played about 2-3 sessions before it ended, with the final fight victorious but the tiefling dying. It was in the Underdark, so it didn’t matter much that the character was unusual, and we didn’t play him long enough to level up.
(3) In a new campaign with new players, that turned out to be a one-shot, a player created a lizard creature I didn’t know from 2e with lots of special abilities. That added to like 3-4 ECL. I allowed it in a 1st level party because his heart was set on it, and he claimed to be an experienced player down for the roleplaying downsides. In that session, he got annoyed that captured armor from orcs I ruled didn’t fit him (he had a tail) and I said needed to be altered by an armorer at a cost before he could use it, and that local villagers were afraid of him and didn’t want to let him into a fortification. He also decided to eat some humans who the orcs had killed, and his girlfriend’s half-elf joined in, to the disgust of other players, so … that “campaign” was a wet squib. More player issue than the monster concept, I think.
(4) In the Sunless Citadel, the PCs adopted Meepo the kobold sorcerer, as an NPC party member/mascot. They had fun with “no kobolds allowed” in Oakhurst, sneaking him in under a sack and hiding him in the barn, “ET” style, and they helped him become king of the Sunless Citadel, at peace with the locals. That - King Meepo of the peaceful magic apple trading Sunless Citadel - is still part of my instance of Greyhawk, in the 2 current campaigns, and it happened in like Session 4 (2nd adventure) of campaign #2 above. Monster races to Alpha and Omega in that campaign, which was something like 38 4-hour games over about 12 years.