There are many invertebrates that have appeared throughout Happy Tree Friends, and they usually make a very small appearance and a minor one at that. However, some play a pivotal role in the episode in which they appeared in.
Insects and Arachnids
The aphid is seen co-existing with the ants in Tongue in Cheek, where it's being milked like a cow by the brother ant. It most likely died when Sniffles' robot ant blew up the anthill.
- Main article: The Ants
The ants are reoccurring villains who are known for killing Sniffles whenever he attempts to eat them. They star in Crazy Ant-ics, Tongue Twister Trouble, A Hard Act to Swallow, Suck It Up, and Tongue in Cheek. They also make a small cameo in Blast from the Past. They appear to be the only insects in the series to have anthropomorphic traits.
Bees and Wasps
The bees make their first appearance in Sweet Ride where they kill Nutty when his head gets stuck in a beehive. They appear again in Take a Hike this time showing no hostility to Nutty when he is slurping honey from a hive. In Aw, Shucks!, a beehive is knocked off it's branch and some bees swarm around Toothy (though these might be wasps since Toothy didn't get an allergic reaction to their stings). One bee appears in The Carpal Tunnel of Love and it stings Toothy in the eye, causing it to get infected. It comes back later and kills Lumpy when it stings his eye, causing him to crash into the back of a truck.
Butterflies and Moths
A butterfly was killed by a carnivorous plant in Idol Curiosity. In Mime to Five a moth flies out of Mime's pockets when he's checking to see if he has any money. Some butterflies are seen flying by in Every Litter Bit Hurts. Flippy might like butterflies because he sees them in his dream in Double Whammy Part I and one butterfly was enough to make Fliqpy snap back to Flippy in By The Seat Of Your Pants.
Flies have made a few appearences in the show, they are mostly seen around dead or dying characters. Flies can be seen flying around the bodies of Cuddles, Toothy, Petunia, and Sniffles in Take a Hike. In Blind Date, flies start swarming around Lumpy's rotting head. They are also seen getting splattered against Lumpy's car in Junk in the Trunk.
Maggots make minor appearances in Remains to be Seen, they are seen crawling on the zombified characters.
Spiders have made several appearances in the HTF world. A spider can be seen crawling inside Lumpy's empty water canteen in Take a Hike. A spider is seen hanging from a web in Nutty's cabinet in Chew Said a Mouthful and there's even a spider living in Lumpy's head in Hear Today, Gone Tomorrow.
Clams and Mussels
A clam is eaten by Russell in Russell's season 1 starring pop-up. Russell can be seen eating some of these in Whose Line Is It Anyway?. Russell picks up a clam in Off the Hook with the intention of eating it but he is crushed by an anchor. A lot of clams appear in Sea What I Found
The Giant Squid
- Main article: The Giant Squid
The Giant Squid appeared first in Doggone It. It was a pet until it attacked and killed several characters before being captured by Lumpy. It appears again in You're Kraken Me Up, causing more carnage after Russell accidentally awakens it by dropping an anchor on its head.
A green crab can be seen flying out of Sniffles' mouth in Mime to Five.
This small blue crab appears in Wipe Out!. It is first seen replacing its old shell with an ice cream cone, nearly being stepped on by Lumpy in the process. At the end of the episode, it uses Flaky's severed head as its new shell.
The Giant Crab
- Main article: The Giant Crab
The sea urchin appears in Wipe Out! where Nutty accidentally mistakes it for ice cream. The urchin's spines pierce through Nutty's head when he tries to eat it, before subsequently drowning in a tide pool.
- Main article: The Demon
The demon is a monster from hell that possesses Cub and kills Petunia in Read 'em and Weep. Since it's a demon, one would expect the creature to have a more ghostly or devil-like appearance, instead it looks more like a cephalopod.
A small jellyfish appears in Happy Trails Pt. 2: Jumping the Shark, it stings Giggles' foot and she later dies since no one could treat it. Additionally, a lot of jellyfish can be seen swimming in groups in Sea What I Found.