Another monster from the Cthulhu Mythos, though this one was created by Frank Belknap Long. All the usual things we’ve sad about Lovecraftian baddies apply here: their alien- and just-plain-wrong-ness, their ties to powers beyond reality and/or other dimensions, and so on. In particular, the hounds hunt those who disturb the fourth dimension of time.
Another thematic angle to explore is that hounds of Tindalos are a consequence of civilization—they can only enter through the artificial angles of mortal architecture rather than the chaos of nature, and they only plague societies sophisticated enough to support the esoteric art of chronomancy. They could be a reason why the ancient high elves never built large metropolises, for instance, or why a decadent but mathematically gifted empire fell.
Also, 3.5 fans might want to set up a rivalry between hounds of Tindalos and beholders. The beholders’ various eyebeams and the hounds’ ripping gazes conjure up images of horrible battles where monsters rip each other apart with powers that warp reality itself. And hounds would loathe the perfectly smooth and cylindrical/spherical lairs created by beholders’ disintegrate beams.
But for GMs, the most interesting thing about the hounds is that they give you an excuse to throw geometry out the window. The threat of hounds of Tindalos (and the architecture their victims use to thwart them) are a permission slip to pull out all the stops: spherical rooms, dungeons built like Möbius strips and Klein bottles, and monasteries out of M. C. Escher prints. If you’ve ever wanted to drop in a 1e-style tesseract or a hyperbolic room design from your calc textbook, the hounds give you all the exigence you need.
The name Titus the Young is spoken of in harsh whispers by other specialist wizards and magi, as one of the few men to have ever mastered chronomancy. His tower, a strange construction resembling a stack of children’s jacks, is reputed to hold great secrets, but the last magus who tried came back with his half his organs sliced open, seemingly by a razor of air.
A ninja dojo contains an odd room shaped like a giant spherical paper lantern, only made out of silk curtains. Inside is a hound of Tindalos, frothing mad at its long captivity in the curved space. If the lantern is entered or torn, the hound attacks. Captured long ago, the hound is the ninja clan’s weapon of last resort, and each clan member carries an enchanted scrap of origami paper enspelled with a version of dominate monster and rope trick that will allow them to recapture the hound should it escape.
A gillwoman becomes horribly mangled in a trawler’s net. The kind fishermen bring her to land to be tended by the clerics, as she will need several days’ worth of healing. Rather than be grateful, the horrified amphibious woman begs to be returned to the “liquid, ever-moving sea.” She says that she is not safe, that she has transgressions from long ago—and, oddly, from far in the future—that she must pay for, and that there are things lurking inside the sunflower-colored walls of her hospice.
—Pathfinder Adventure Path 4 82–83 & Pathfinder Bestiary 3 158
A female gillman? Yeah, you saw what I did there.
This is one monster it’s worth doing more research on. I’m guessing Chaosism’s various Call of Cthulhu products have more, and the full monster description in Pathfinder Adventure Path 4: Fortress of the Stone Giants is worth checking out.