Pox Hounds in Darktide move around quickly and erratically, and their goal is to pounce on an isolated party member. The pounced player cannot break free and will take continuous corrupting damage until freed. You will hear when the Pox Hound spawns due to a howl in the distance, and it should always be a priority target as soon as it's in range.
You can dodge to the side to avoid the pounce of the Pox Hound - it just takes a bit of practice to get the timing right. The audio cue for dodging the Hound's pounce is a high pitched bark that it always emits as it begins to pounce. Dodge sideways when you hear this.