The bullsquid is a fictional alien creature that you encounter repeatedly in all the video games of the Half-Life series (So far, this includes Half-Life, Half-Life: Opposing Force, Half-Life: Blue Shift, and Half-Life 2).
The bullsquid is an amphibious monster, found in and around the rivers, ponds, and other wet areas of the alien world called Xen. They also have the (infrequent) tendency to hide in tight crevaces.
They are immune to, and even enjoy hanging around in, radioactive waste.
They are approximately 2 metres in length, and are a yellow-ish colour with dark brown and olive spots, like those on a toad. The basic shape of the bullsquid's body is that of a squid, and has two cloven-hooved legs, like those of a bull. Bull-squid, get it? The front end has two large frog or squid-like eyes, and a fanged mouth ringed by five short claw-tipped tentacles. This tentacle area is always very red, either from natural colouration or the blood of it's last meal. The other end features a club tail tipped with a non-poisonous scorpion-spike.
The bullsquid sometimes attacks by ramming its prey (like a bull would), and then using a 360 degree spin to smack it with its spiked tail. However, it prefers to attack long-range, by spitting gobs of acidic green slime out of its mouth.
These globs can easily travel the length of a warehouse with reasonable accuracy. Luckily, bullsquid aren't very smart.
The bullsquid is an unfriendly chap, and it will attack most everything it sees. It's prey includes the headcrab, humanity and, sometimes, other bullsquid. Interestingly, one of the only things it will not kill is the non-threatening houndeye.
When not killing, bullsquids love to dunk their heads in tastier liquids and drink for hours. Their favorite drinks include the glowing blue liquid found in Xen's healing pools and the blood of well-crushed prey.
Unlike some other Xen creatures, the bullsquid has no complex societal structure or any group behaviour at all, for that matter. The only time that they're not going to try to kill you is when they are busy drinking the night away.